Going to cut axle tubes

Hello I was wondering if anyone on here can help me out with a bit of info ? I have a 71 Plymouth Duster which I have done a mini tub job and now I'm looking at putting axles in. I'm thinking of cutting my rear end since I have also moved my spings under the frame ( rear rails ) The rear end I'm useing is a 8 3/4 and was wondering how much to cut off the axle tubes and how much shorter does the axles have to be when I order them ? Is there a way to measure this or a formula for this answer ?

Thanks for any advice and help:cheers: