In your own words, cleaned up for the kitchen wall. This explains why 1.2 base recipes — one full batch plus a small top-up — go into every vanilla 3-gallon tub, and why that number is what the cost engine now uses.
The first version of this guide worked from an early description ("1 full batch + 2/3 of another") and concluded 2 full recipes per tub. Once you gave the precise gallon numbers — one batch churns to 2.5 gallons on its own, and the tub needs the last half gallon to fill — the math changed. 1 + (0.5 ÷ 2.5) = 1.2 batches per tub, not 2. This page and the cost engine both now use the corrected number.
One batch churned by itself makes 2.5 gallons. A tub needs 3. The gap is a half gallon — not another whole batch.
Because each batch makes more (2.5 gal) than a single top-up needs (0.5 gal), the leftover carries most of the way through the next tub too. Averaged over many tubs, it still works out to one full batch plus a fifth of another — 1.2 batches per tub — the exact scheduling (whether that's one fresh batch every tub or one fresh batch every few tubs) doesn't change that average, only how it feels day to day in the kitchen.
The cost engine asks "this batch yields ___ tubs." One batch (2.5 gal) covers 2.5÷3 = 0.8333 tubs on its own — that's the number now pre-filled for Vanilla. It's the reciprocal of 1.2 batches per tub, same fact either way round.
You said it yourself: what fits in the machine varies by flavor. Cookie pieces, fruit, ribbons and chips all take up room that would otherwise be liquid mix, so a loaded flavor may need less top-up than the plain base does — or the churned yield of a single batch may differ once inclusions are folded in.
Treat 0.8333 tubs per batch (1.2 batches per tub) as the vanilla baseline only. Confirm the real churned yield for each new flavor as you measure it — the cost engine now gives every flavor its own batch-yield field for exactly this reason.