Advancing creatures and new CR

[Corwyn]

I'm Dming RtToEE and the party consists of 6 players and one npc.

To take this in account I adjusted an encounter by advancing the creature from 10 HD to 15 HD as indicated in the MM from large to large.
My question at this moment is with how much did the CR go up??
Just with +1 ? or is it more ?

 

[Dr. Zoom]

The CR increases by 1 for every 50% increase in HD. 10 HD to 15 HD is exactly a 50% increase, so the CR increases by 1.

 

[Lord Ben]

Sometimes common sense takes over and you realize it would change by more then that. Sometimes by less then that.