You are right.
SIN function will tell you the windage/elevation error caused by the corresponding elevation/windage holds/dials.
Increasing the cant angle increases the error trigonometrically
Increasing the hold/dial increases the error linearly
And yes, generally the elevation hold/dial is greater than the wind hold/dial, so its corresponding error is greater (error will be seen as a windage miss)
To measure the resolution of the bubble level graduations,
- set the level on top of a straight edge of known length (like a 48" carpenter level for example. You are only using it as a straight edge though, don't worry about what the level's bubble shows. just watch the UM level.)
- Shim the very outside edges of the straight edge until the UM level shows perfectly centered. This is now your starting point. Don't record the thickness of these shims.
- Add shims (to either side, doesn't matter), until the UM's bubble is just touching the line. Record the thickness of ADDED shims.
Now the following calc will determine what angle it took to move the UM bubble to the line. Make sure your calculator is set to "degrees" if you want degrees:
TAN^-1(ADDED shim thickness / straight edge length)
A more precise way to do it would be to start with the bubble touching one line, then shim until it touches the other line. Do the same equation, and then divide by 2