When you are low on cap? I mean, if you are around 25% cap, use the booster with a charge that either fills you up to full (that’s a 800 in a MCB or a 400 in a SCB) or start running your cap booster with size appropriate charges early on to keep the cap up until the target is dead.
That’s wholly irrelevant. You do not rely on cap recharge if you use a cap booster. Worrying about recharge when you use a Booster is a waste of time and brain capacity. Your booster basically is your 5000% regeneration cheat.