You have to prep the plates for counter sunk screws. A simple centering reamer bit does that.
I've never have any issues with the plates altering their lateral position. I've done this countless times.
Also about a formula. While you could base one on the maximum CFM draw of the engine based on size at 100% VE that would not be correct at all.
This formula would have to take into account more variables then just that and even then it's only based on optimum conditions.
That was the reason I posted the all the information about other factors being more restrictive then the TB. If other factors are more restrictive then the TB then enlarging it will only hurt velocity at the inlet of the manifolds.
In dyno and real world data it shows that a 65mm TB loses power verses an optimized 60mm TB on even moderately modified SVTs.
For that matter some dynos have shown no gains swapping to an SVT TB on an otherwise lightly modified engine.
The variables are too great to have a simple equation to be used as a basis for choosing a TB. Even if you did have one it would likely have a high range of error comparatively. (even a 3% error is +/- 2mm in TB size)
As for your "quality" of airflow. That is why an optimized TB shows improvement. It's not because the actual flow area is increased.
There are books on how to model, size, and design throttle bodies. You should try PMing Rara. He's sure to have knowledge or friends that know about it.
