So - You want to learn to fly?

You forgot to 'insert' your photo''s!
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I know nothing of raspberry pi's, and in my mind, a python is a kind of snake - so I don't have a clue what you are trying to achieve or prove.
Until you have a transmitter and matching receiver, you won't get anywhere trying to set control throws up - it all depends on the particular transmitter/reciever/servo combination!
As Quorneng put, you will only get a 90' movement (45 each way) - end to end - with a servo connected to a receiver.
Those little translucent blue towerpro servos are not the best to start with, and minor variations in quality control and component tolerances can give minor positional errors anyway. (we say you can get a bag of ten of those for a fiver!) They are only suitable for light duties. I use one as a sacrificial servo for a steering nose leg, so that if it gets a hard knock it just breaks THAT servo, and not the main one controlling the rudder, which is much more important!
That's what the sub-trim that Klippy mentioned is for - correcting minor centreing errors. The trimmer sliders at the sides of the sticks on the transmitter you haven't yet got are for minor adjustments to the control surface IN FLIGHT to achieve straight and level hands-off. Those sliders should be set to central or zero, before you start trying to connect the surfaces to the servos.
Those Towerpro (and copy) servos have a spline on the output shaft - that the arm fits on to - it has matching splines on the inside of the centre, where the screw-hole is. Some servos have a square block for the output, and no splines. That is so that the arm cannot slip - that could cause problems for the pilot, and fun for the spectators!

The photos are little thumbnails at the bottom of the post, last time I posted photos there was two of each so I pressed less buttons this time. I'll see if I can get back into the post & try again.
The point of using a Raspberry Pi is so I can play around & understand how the servos work. It gives people an opportunity to try programming themselves instead of just buying an off-the-shelf windows job - a bit like people tinkering with a car. (I'll post photos of our Triumph Stag & Ford Capri at a future date.)
Python is just a programming language but its not the nicest one I've worked with, a few years ago I did some Android programming, that was just as bad.
Once I get to owning a transmitter I should have a better understanding of what's happening.
The SG90's have a nominal rotation of 180 deg, again being a newbie I thought that was the norm.
They are sold by what I believe is a reputable firm, and are widely used in education, or so I gather. Maybe cos they're cheap it doesn't matter if the students break them.

You can see the photos if you click on them, I went back in but couldn't insert them in the text.

I think you have about ten minutes after posting, if you want to go back in and edit, after that it is 'stuck'.
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I know the thumbnails will open by clicking on them. As I'm a moderator, I'm supposed to be able to correct things like this, but I'm still figuring out how it works, myself!  

Photos inserted, I understand you trying to reinvent the wheel, great stuff, it makes us who have moved on think about our journey to here.
As a petrol head I like the mention of the Stag and the Capri (crappy)
If you search around on here you'll find a thread by Boz on his Austin A30 (35?)
For myself I can admit to rebuilding a 1974 Mini GTS (local Mini Cooper 1275), 1961 MG Midget and a 1971 Rolls Royce Silver Shadow none of which I still own, there comes an age where you no longer enjoy crawling around in a pit underneath cars. And the Rolls nearly killed me.

Modern cars are carefully designed to be almost impossible to home-repair!
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Magpie - if you ignore the rotational movement of a servo arm, but instead attach a pushrod to it, then observe how far the other end of rod moves - in a linear fashion, you will find it's greatest movement is with the servo arm at the mid point - to about 45' either way, after that the linear movement is much less, and the last ten degrees of the 180 arc of the servo moves the other end of the pushrod hardly at all!

Ah the Triumph Stag!
As an tool room apprentice at J Sankey and Sons in Shropshire, who made most of the tooling for Triumph, I was given the job of polishing the surface of the press tool for the Stag boot lid after it had been milled to shape on a huge Pratt & Whitney "Keller" copy milling machine from a wooden pattern made in the carpenter's shop. No NC back then!  The foreman gave me a carborundum stone and said polish out all the milling marks.. It took 3 days before the surface met his satisfaction but that was what tool room apprentices were for. I have a particular feeling about the Stag boot lid shape. 
"Stag" was Triumph's code name for the car. Very unusual for it to end up as the car's actual name. Herald was "Zobo", Spitfire was "Bomb". Dolomite was "Ajax".
I owned a Dolomite Sprint and a genuine Cosworth modified Lotus 7 but that's another story. 

Eric
I understand what you're are saying about the relationship between servo angle & push rod travel. plus if the servo & pushrod are not well aligned the last degrees of rotation will be problematic.

My son does most of the physical work on the cars, increasingly so. Its not so much getting down & under, its getting out & up again that's my problem!
I don't care for the capri, its a struggle getting in & out now & its like driving a bus but its the love of his life, ( his wife sometimes says she will cite it for a divorce).
I suggested he got the Stag after he failed to buy some other triumph on ebay. We didn't dare tell my wife we'd bought it until the pick-up truck arrived with it on the back.
The boot is the worst part of the car, after changing the radiator & a new one wouldn't fit I realised it had been in a shunt.
The best car I ever had was a Triumph 1300 TC, beautifull but I could never get the carbs balanced.

My Landrover 110 V8 had the same problem, I could never get it to run or idle nicely until I changed the twin Strombergs to twin SU's.