Offset is measured from the midline of the rim, so 0mm would have the mating surface for the hub be right in the middle.
Backspace is the distance from the rear face of the rim to the mounting surface... again, if it were a 0mm offset, the Backspace dimension would be half the rim width + 0.5" to account for the inboard flange. (offset is added or subtracted from that number otherwise)
If you have a 6.5" rim (165.1mm) with a +37mm offset, then your backspacing is half of 165.1 (82.55mm) + 37mm of offset = 119.55mm or 4.7" + the 0.5" flange, grand total, 5.2", either way, only one of those (offset vs backspace) is needed to know where a wheel's going to sit.
The stock offset for the 5.5" rims (if memory serves... and it's what I've been basing everything off of for 2 years so... that would be embarrassing lol) have a 55mm offset, the Autoshow release had them on 18"s with 42mm to have them "flush".
Here's stock vs your proposed setup - with straight lines it does show as passing the lip of the fender, but tires do curve back, and if you're not lowered you'd have to take one hell of a bump, likely while turning, before you'd have to worry about rubbing. If the front does need to be adjusted though, speaking from experience, a gentle tug on the front fenders fixes a lot - mine are at about 30 degrees, took 10 minutes.
also shown is the output from willtheyfit for speedo error reference.
View my fuel log 2015 Micra SV automatic: 7.0 L/100 km ... 40.6 mpg (Imp) ... 14.4 km/L ... 33.8 mpg (US) ...