Have some backspacing questions

I want to get some new wheels but I want to move away from my stock 96 mustang gt wheels plus spacer. my problem is I like where the wheels sit in the rear with the spacer and can't understand what my new wheels backspacing should be. I'm looking at Vintage 40s with a 0mm offset and a 4.5in backspacing. I know the gt wheels are a 5.72in backspacing with a +30mm offset. to keep the outside of the wheels the same is 4.5in backspacing right?

Can someone help me understand this? maybe show your math?

thanks for your time Grant