Horlicks, the pressure will be higher by quite a margin, the regiis there ot ensure that enough pressure is ther when you suddenly boot it and also to give a known pressure so that the relevant injector phase opening time can occur to deliver enough fuel.
If it was 3.5 bar from pump it would drop a lot upon WOT, albeit maybe breifly but enough to do damage, the reason they use a regulator and not do without, which they could if it just ran at 3.5 bar alll the time !!.
You need to measure at the rail as this is the one that matters for your engine, it it was 1,000000 bar before it would not tell you anything or matter, as it is regulated to 3.5 bar , so long as you see correct amount you KNOW PUMP IS WORKING OK, and that the reg is working ok.
tabetha