I've asked this same question in the RAW thread before many moons ago: there is no clear answer.
I think what White Blade is asking, to give an example, would be does a Half-fiend Beholder's CR increase by +1 per 2 RHD (as per Outsider), or +1 per 4 RHD (as Aberration, it's base creature type).
It's probably best to use your judgement, depending how much the creature is benefiting from it's RHD.
Yeah, that's what I was asking. Thank you for the answer.
Vincent Omnia Veritas
Bandwagon Leader of the Hinjo Fanclub