Hi Mary Ann,
That +2 comment applies to Lantus only, and only to some cats. It's because Lantus typically has an onset of about 2 hours in most cats, so if the cat is dropping BEFORE the Lantus has onset, then that's a red flag for the person to be more on top of things than usual. There are some cats that typically have a lower +1 and +2 than their preshot, and that's just their pattern, so for them the comment you quoted above wouldn't apply.
With Levemir, the typical onset is about 4 hours, meaning that it begins to work to lower the blood sugar starting about 4 hours after the shot. So the +2 is basically meaningless. In that case, it would be the +4 that would be more significant.
But these are generalizations that might or might not apply to Tuxie. You'll have to look at his cycles and see if you can discern when you can see the Lev beginning to work to lower his blood sugar. Some cats on Lev have their nadirs around shot time, so roughly 8 hours after the Lev may have onset. That's one reason why those on Lev have to become used to shooting lower numbers, because they are sometimes shooting the very lowest number of the cycle, although that shot being given might not start doing anything for 4 or even more hours after it's given. You're shooting that distant number hours away, not the test you've got right when you are shooting.
Your question about what is a fast drop - certainly anything over 50 point per hour would be considered fast. It's not a percentage, but a flat #. Cats that have their insulin onset from a lower number won't usually drop as fast or as far as a cat starting from a higher number. The difference between shooting a high number and a low number is much like the difference between playing regular golf (the ball goes high and far) and mini golf (the ball stays mostly flat and doesn't go very far.) Both Lantus & Lev have the ability to give a flat cycle - one that might only move 10 points in the entire 12 hours. That comes from keeping the cat's blood sugar tightly regulated in normal numbers.