More experienced people correct me if I’m wrong:
Manual Stretches - 0-10% (I just say 10% for a number, though the more blood in the penis the less you can really stretch it no?)
Jelqs - 50-80%
Kegels don’t actually have anything to do with how erect you are and I haven’t exactly heard any positives from doing them erect so 0-100%.
Edit: Now that I think about it, it really depends on the stretch. When I think Manual I think of what you usually would do in a warm-up routine, but I mean something like an A stretch or V stretch (generally regarded something non-newbies should do) I like to do probably 50%. So I am pretty much of length but still rather limp.