How to make servo rotate in a given range no matter how its placed in a chassis
This is a typical RC servo problem. You need to set home position on your servo by simply building a 1.5ms 20 fps servo driver, or use a servo calibrator, or write it into your software. 

Activate the servo, and it will now hold it's position and use a simple jig to help attach each joint (this could be as simple as a paper indicator). 

Trying to build a multiple servo device when the servos are unpowered is not the way to go.