90 deg to the arm may work for 99.99% of the world but it's never worked properly for me.

Get it close by eyeball to start with, then adjust the servo center with the subtrims and pushrod length until you get the same deflection each way with the endpoints set at 100%. Then, there's a really good chance that if you then set your endpoints back to 25% you'll still get equal deflections each way.

The linkage geometry we use to reduce +- 45 degrees of servo movent down to +- 15 degrees of surface deflection actually adds exponential to the surface travel making it more responsive around neutral, the opposite of what we usually want. So adding 10% more endpoint to get the deflection equal (for example left aileron 100%up and 110% down = +- 15 degrees at 100% rates) will, when you use low rates eg 50%, yield something like +9deg, -10deg.

Sounds confusing, but it's all part of the fun of setting up a plane mechanically before you reach for the electronic fixes.