I’m double-checking, after the recent thread about decanting. Can I assume that the recommended size of a yeast starter is based on the size of its wort prior to fermentation, not on the slurry remaining after decanting?
Yes, that is correct.
The amount of yeast is what matters; whether it ferments out and you decant it or you pitch the entire active starter is of little relevance (as long as the large starter volume doesn’t alter the final beer anyway).
Veldy