amdoming5

I have axles and calipers for a rear disc conversion that I bought from ebay a couple of years ago. I also have some 2001 gt wheels that I bought for the conversion. I read a few days ago that there are different length axles and depending on the length, it may or may not work with the wheel backspacing. I understand the sn95 axles are 3/4" longer, but I don't know the stock length or sn95 length so I can measure the ones I have and find out what I have. Also where is it measure from?
Thanks for the help.