I've got the ARF version and I'm now on my 3rd engine.....
I'm not sure if I measured it when I first put the Irvine 39 in it, but I don't think I bothered when I fitted the OS46FX and as for the OS FS70 I recently put in I just put it as close to the firewall as possible.

With an uncowled engine, unless there are balance problems and you have a fair amount of leeway, I'd just mount it so you have sufficient clearance in front of the firewall to route the fuel tubing and you have around 1/4" clearance in front of the side cheeks....

I guess as engine sizes vary, SIG just choose to specify this as distance from firewall to prop. This is not critical.