Depends on you. I'd normally start with about a gram... I've eaten many grams of 30% and 84% paste before, and it's generally pretty safe on its own. I've never combined it with alcohol, but I hear this is pretty popular. In that case you'd probably wanna dose lower. I'd suggest starting out with a gram or two to get the feel for it, before you experiment with higher doses/combos. Don't expect a super sedative buzz or anything like that, more like a mild anti anxiety feeling, a light kind of blissfulness.
Enjoy.
p.s. Here is a trip report about kava paste, albeit it's a higher concentration. It'll give you a feel for what you're info generally though. Search erowid for more trip reports to future explore dosing/combo opportunities.