I use "Lord" for any male noble without a position. I only use "Sir" if they've donated. Occasionally someone objects to being called a Lord if they're not a region lord, so I refer to them as "Knight" instead. Not sure what I'd call a male noble without an estate if they object to being called Lord.
"Knight" isn't generally appropriate as a prefixed title. The prefixed title you use for a knight is "Sir". That's...just the way it really was. (Unless someone can find me some countervailing evidence...)
Like I said before, donation has no real IC significance, so I don't feel it's appropriate to make an IC distinction between people who have donated and people who haven't.
