I'm a nerd and since I've ruined festivals for myself when I didn't plan well for battery power, I'm extra conservative when it comes to planning my battery situation out.
To make sure I have enough power for a full weekend, I hook everything up weeks beforehand and let it run dead to get run-times with the batteries I plan on using. That gives me alot more confidence than doing a calculation about what you should get based on the battery mah rating. It also cycles your batteries.
When the festival arrives, I plan battery change-outs based on these run-times. For example, if I know that I only have 1 hour left on a battery and a 1 1/2 hour set is coming up, it might be a good time for swap. I carry extra batteries to account for failures or mistakes. This method gives me 100% assurance I'll get through the weekend.
Hint: In additional to knowing the amount of time to expect from your batteries, using external batteries supplied by a cable that allows for hot swaps also helps maximize battery usage, while minimizing the potential for recording downtime.