First part is specific to lantus, written by Chris & China. Second part is more specific to Prozinc, written by me.
With lantus from Chris&China
The +2 is like an "early warning" for most cats!
If the +2 is about the same as the PS, it's usually a pretty normal cycle...dropping down to nadir and then rising back up again.
If the +2 is higher than the PS, that usually signals a bounce, and those are the cycles when you can usually take a break from testing.
If the +2 is lower than the PS, that's your "Early warning" that kitty might be going much lower later in the cycle so it's important to plan on getting another test or two (or 6) in.
It doesn’t work every time or with every cat. But it does hold true most of the time, so keep this in mind for next time.
With Prozinc by Deb & Wink
More than a number range, think of the percentage of the drop at +2, that may tell you if your cat is going to drop much lower. Think of that +2 as being an indicator of what direction the BG levels may be headed.
With Prozinc, if you see a 40%, 50% or greater drop, between the pre-shot and that +2, that's a serious heads up warning.
If you see a 20-25%, 30% drop, not quite so worrisome. But still be cautious.
Your cat drops 2 color ranges between pre-shot and the +2, pull out your hypo kit and plan on staying up to test.
Your cat drops 2 color ranges between pre-shot and the nadir, that is to be expected. But you don't want the drop to be much more than 50% between pre-shot and nadir with Prozinc.
Cats tend to bounce high if the drop is 50% or more from pre-shot to nadir.
So you steer those mid-cycles with food, to slow down the steep drop and stop the bounces from being too high.
It's a balancing act.
The +2 is like an "early warning" for most cats!
If the +2 is about the same as the PS, it's usually a pretty normal cycle...dropping down to nadir and then rising back up again. (About the same means taking meter variance, that 15% variance that hand held glucometers are allowed to have into account)
If the +2 is higher than the PS, that usually signals a bounce, and those are the cycles when you can usually take a break from testing.
If the +2 is lower than the PS, that's your "Early warning" that kitty might be going much lower later in the cycle so it's important to plan on getting another test or two (or 6) in.
It doesn’t work every time or with every cat. But it does hold true most of the time, so keep this in mind for next time.
It also kind of depends on what BG pre-shot your cat started out the cycle with. Lower than normal, keep a closer eye on the cycle. Lower than you are used to shooting, a number you may not have given insulin before, keep a closer eye on the cycle. Stalled and number rose some, but you did a reduced shot, maybe another test or 2 to see how much the number is rising.
It makes a big difference if your cat starts the cycle in the pink 300-399 mg/dL range (16.6 - 22.16 mmol/L) then if they are in the blue 150 mg/dL range ( 8.3 mmol/L).
p.s. Food notes in the Remarks column can be very useful. Helps to interpret what is happening in the cycle. Notes are done on AM and PM cycle basis and notations need to be with the + time format.
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, or the meter slips out of your hand and goes "Kerplunk" from 4 feet up onto a hard tile floor. 
