whenever I transfer from right to left side it works but it seems like zero is lower on one side of the car than the other
That’s because it IS… welcome to the delights of painting in Forza, LoL! 
There’s a thread just below this one that might help you line up your decals/vinyl groups… click HERE.
figured it out using the guide you mentioned used that method to do some quick math with my handy dandy calculator and boom I think I got this issue licked