Sorry for the double post, just trying to bump after my unnecessary discouragement.
On second thought, you're probably using a small mass of lye (like ≤ 50g or so), so that would probably saponify somewhere around less than a pound of triglycerides. So it really wouldn't waste that much oil even if you ended up not using the product. Maybe you could use the pH to estimate... or just take a guess. Saponification may take a day or two at room temperature (i.e. cold process), and only then will you be able to evaluate excess fat or lye... after it has already turned hard. So you might have to reprocess it to adjust if required.
As an aside: I just used some rancid olive oil for soap. Way too much bar soap now that most other people wouldn't want... no fragrance and it looks very homemade, lol. Pretty easy, quick and inexpensive though.