I find that for lighter blonds having regrowth foils done who want a brighter result overall & to avoid lighter regrowth with a darker drab remaining length of said regrowth foils.......it is good to use 20vol+bleach for roots from front hairline-top through the crown where hair is more finer textured & takes the abuse of wear & tare....then I always use a gentler bleach with 10vol like Farouks CHI Bleach & apply on/in same foil as root with stronger bleach formula, the gentler weaker bleaching product through out the previously lightened darker lengths. Use a sweeping, slightly overlapping brush stroke downward into the blond lengths & a sweeping slightly upward/backward sweep ( against hair grain) in to the overlaping stronger bleach sweep. Sweep the lengths of the remaining darker blond lengths in foil towards the ends without getting bleach too heavy or any at all when you reach just before the porous ends or till where hair is more porous near ends with gentler bleach. All in same foil!
Tip: when top & sides to lower crown is all foiled..... mix up a fresh bleach+30vol for root retouch & some fresh 10vol+gentler bleach for back section of the hair( hair is stronger & darker here & 25-30mins from finishing full foils application, the back will be matched better to the front bleached regrowth) If hair is on the more fragile side for front & top......check first few rows of foils for lift & evenness. If these first foils are done by the time your full foils are finally done.....just damp rag with a little conditioner mixed in to the rag, off the bulk of the bleach of those first placed done foils (T zone usually).
Handy when you have a big head-O-Foils on dense hair types!...... Tone all over with 10vol or less.