Since this is NANOG, I'll restrict my comments to ARIN policy:
Unfortunately RFC2050 doesn't address at what point additional address space should be assigned to an end user, however, it never uses the 80% figure in that respect.
Interestingly, ARIN doesn't publish any policy on additional address space assignments to end-users. However, I can tell you that in practice (and it's common sense, too) ARIN does not issue additional assignments to end-users until they demonstrate that they have used their previous assignment efficiently (80%). Why? Because:
This means that if they are already at 50%, you assign them another block equial in size to what they have, and they reasonably expect to double their utilization in the next year, that everyone should be happy.
You can't use 25% of the additional block immediately if you still have 50% of the initial block available*. Again, though it's not written down (Richard Jimmerson?? Comments?), ARIN does not issue end-users additional blocks until their existing blocks are efficiently utilized. /david [*] if you can, you're a special case that's not material to this discussion.