It's very individual for me.
Before we even think about adding a BHS to a roundoff, I want it long, tight, proper block, head in--technically strong. This takes a lot of work, esp if a kid has a "roundoff" that needs fixing.
As far as doing the BHS goes, I want it to be tight and consistent from a stand on TT, from a jump to angle on TT, into a squishy off TT, and on the floor. That doesn't mean it has to be 100% independent, but it does need to be consistent and have good shapes.
Before adding the RO to the BHS I like to do angle BHS, snapdown BHS, and 2 BHS in a row--with or without a spot. These drills are all easier to correct if something goes wonky & use many of the same shapes and actions as RO BHS.