Question about replacing 15x7 wheels with 15x8

My car currently has 15x7 rallys with 4 1/4" backspacing and 255-60 tires on the rear. I want to get some aftermarket wheels that are 15x8 and still use the 255-60 tires. In doing so I want to keep the tire in the exact same position in the wheelwell as it is now. My thinking is that to position the tire in the same spot in the wheelwell as it is on the 15x7 with the 4 1/4 BS that I will need the 15x8 to have 4 3/4 BS. I just figured that dividing the extra 1" inch of rim width in half (half to the inside and half to the outside) and adding it to the 4 1/4 BS of the 7" rim would put me at 4 3/4" BS on the 8" rim. Does this sound right?