The tyre's height is percent of its width is expressed as the 255/40 so the actual height of the tyre from the road to the rim will vary on the width. My Porsche and GTR both have different percentages front to back. It's not a problem, sticking to the same value will mean you will have different tyre height road to the rim!It’s a tricky one and glad I’m not the only one having the problem.. I’d rather not change profiles between front and back if I can help it and the wheels will very likely be wider at the back so it’s just trying to find the size that suits.
After looking more today it seems you can get hold of the R888R in some wider sizes such as 275/40/17 (despite what their website says..?) along with 255 and 235. If I were to go 16 they do a 255/50/16 with 245 and 225. The extra profile on the 16s is very tempting for some extra damping and suppleness, like jonbills stated the extra 20mm of width might not be worth the trade off.
Still keen to hear from any of the current high HP RB/JZ owners on what they run and their feedback?