Overusing overpower is usually what gives new WARs issues.

I would say:
Do a few runs being as conservative with overpower as you can. Focus the use of overpower toward keeping threat, not dealing damage. This will give you an idea of how much TP you would have had to play with / how spammy you could have gotten with it.

^ after you've done this, plan the pulls that you really want to go to town on, and make those pulls as large as you can realistically make them. The easiest way to make sure you have enough overpowers to handle 2+ groups of mobs, is to handle all groups at the same time xD.

Lastly, when planning to spam overpower, make sure you're also planning on dmg buffs (you might as well throw down vengeance as well). More dmg per overpower means fewer overpowers are needed to accomplish the same result.