You're not giving us enough information for us to give you the right advice.
If the client is someone you don't have to compete for, and is someone whom you can trust...
It seems to me your best plan is to go "cost plus." That is, figure out how much EVERYTHING, including everyone else's labor, is going to cost, figure out how much time you're going to spend on the project and what YOU (not me, Ed or shroom) think is a reasonable rate for your time.
Then give your client an estimate based on those figures. If necessary, get a deposit in either a partial or in the full amount -- depending on how much you trust the client.
After making your food purchases, let your client know immediately if you have exceeded the estmate for food costs. If you tell him by phone, send a confirmatory fax, email, or text message immediately ("Just to remind you of our conversation earlier this afternoon ..."). Make sure it's something you can print out. ON PAPER ALWAYS -- it makes everything go smoother for both parties.
When the job is done, adjust the figures to reflect the receipts and time slips. Then present your client with a rebate or bill as necessary. Rebates are better -- remember that when you make the estimate.
Given your situation, tripling food costs is a less precise way of doing the same thing -- with a greater tendency to over or undercompensate for labor. Yours in particular. Don't pretend your fee, as a multiple of other costs, is out of your control -- because you're too shy to tell the client how much your worth. I know it's embarassing when you haven't done it in a long time, but figure out how much you should get paid and ask for it forthrightly.
Ex owner operator Predominantly French catering; ex cook at a couple of good joints