How to determine servo signal encoding
Thought I'd put it in an answer since that's all it is. 

Position "0" (1.5 ms pulse) is middle, "90" (~2 ms pulse) is all the way to the right, "-90" (~1ms pulse) is all the way to the left.

Datasheet:  http://www.datasheet4u.com/datasheet-pdf/TowerPro/SG90/pdf.php?id=791970