20 pages (for me, at least), is a rough guide. I'd much rather you send me 24 if it means that I won't have an artificial break in the middle of a chapter. Keep in mind that if I say 20 and you send 24, I might only end up reading 10. If I get to 20 and I'm still interested, then what do the extra four pages mean? Chance are, I'll be requesting the whole thing shortly, unless those four pages are really terrible.
Super glad to see this, because this has been my approach to shorter partials. I teach college, and so I know what "x" number of pages feels like, even when my students have fiddled with margins, font size, etc. Yet I want my partials to have some "flow" to them, so I like them to cut at a natural break (I've just started querying, but I have my 3-page, 5-page, 10-page and 20-page breaks marked in my mss).
I always view a shorter partial, or especially pages with a query as a
amuse bouche. An agent isn't getting the meal, or even the whole first course...just enough to see if the chef is any good. And they've probably decided within a few bites if they want to order supper--if there's an extra bite at the end (or a little bite less), it's probably not going to sway the decisions either way.