Epoxy IS resin, and if I was ordering it, I would get approx 3:1 epoxy resin. I only used 2:1 as an example for buying the dispensing pumps.
By resin, I'm guessing you mean polyester resin?
Polyester resin doesn't seem to 'go' as far because you MUST use a fiberglass mat or cloth of some sort with it, and the fiberglass MUST be fully wetted out. THAT takes quite a bit of resin. If you use fiberglass w/ epoxy, it too won't 'go' as far.
My estimate of needing a gallon is extremely rough. Have you ever worked w/ epoxy or polyester resins before? If not, then you may use more (mixing a larger volume then you need so you can be sure you won't run out) or less then I would. Working too slowly applying it, can also allow the epoxy to start to kick in the container you mixed it in. Then you end up w/ nice modern art hockey pucks:
This is also what happpens to what ever epoxy you have left over ^^^
If you mis-measure either part of the epoxy, or make incorrect judgements about having sufficient working temps, you might have to scrape the tacky epoxy goo off the wood and start over. I know I've done that in the past, working in less then ideal temps. I may have also mis-measured the components. It was certainly NO fun removing the still not cured epoxy resin from my transom. But is just another way you'll use up resin w/ out making any progress, and need more or less then I would.