Unless you are up for getting an engine on a bench dyno you will never know (even then it could be out by a bit)
RR's should be used as a before and after mod test, to give a rough idea if a mod has been beneficial and by how much.
The old leon cupra owners claimed (still do i expect) they could get well above what they actually could (bhp figures) after a remap. One of the more accurate RR's is JKM, this showed them up quite badly. Most cars (standard) that go on there hit the manufacturers figures. I would still never fully trust a RR though, tyre pressure, temp, petrol used all alter the outcome.