Appox. how much should I mix?

[JohnB]

If your using epoxy, I would not thin it, I would use slow hardner on it, and do it in the evening when it is cool. It will take at least 2 coats, and I would wait a day between coats. I use a plastic spreader to evenly apply it, and I think it takes about 12 oz for the first coat, and 8 for the second. You don't want any puddles, just wet it out, light sand between coats.
Leave it facing up, and only do one side at a time. Epoxy migrates MUCH better than poly glass.
This is assuming your using epoxy.

[Islandtrader]

I am using epoxy. Slow hardener yep. Thanks for the night time suggestion.