i wish there was an easy answer to your question about shooting early.

shooting early is
less about being at a high enough number and
more about how well
YOU know
YOUR cat...
YOUR cat's response to the insulin used.
in other words...
- do you know when your cat experiences onset?
- do you know when your cat usually reaches peak/nadir?
- do you know how much duration your cat is getting from this dose?
- do you know YOUR cat well enough to predict what will happen the next cycle?
- will you be available to monitor closely just in case kitty surprises you by dropping lower than expected?
- the next shot will be due 12 hours from whenever you shoot. does this work for you?
if you answer "no" to any of the questions above your best bet is to refrain from shooting early. you don't have enough understanding (be patient - this WILL come in time) or data accumulated
yet to safely shoot early. gradually move your shot times back in increments of 15 - 30 minutes as suggested in the guidelines unless an experienced member who has the ability to analyze spreadsheets is available to offer assistance.
if you can answer "yes" to the
ALL the questions above, you have the information required to make an educated decision as to whether it's safe to shoot early.
the ability to shoot early is all about knowing
YOUR cat's response to the insulin used.
if your kitty is
known to dive early in the cycle, shooting early (even from a high number) may not be the best idea because of the extra overlap/carryover created by shooting early. i get nervous when i see people recommend shooting early just because kitty is at a high number. it's not about the number. it's about that individual cat's response to insulin. kitties do and have been known to take a nose dive from a very high number. this is especially true with kitties known to be "early divers". this can become problematic... even dangerous... because it's not always easy to bring numbers up (even with HC) when you shoot an "early diver" early.
generally speaking, there's less of a problem shooting early with kitties who are known to experience HUGE food spikes from low carb food. if your cat routinely experiences these horrendous food spikes shooting early usually isn't a big deal because you know your cat's numbers will be going up before the insulin will kick in (onset). see alex's spreadsheet for examples. for the last year and a half i've been trying to remember to record shot times. i rarely shoot on a consistent 12/12 schedule because life gets in the way and i have never
purposely moved shots in increments of 15 - 30 minutes. she's currently on levemir. her onset isn't until around +4 and she experiences humongous food spikes. shooting an hour or two early isn't a problem
for her.
having said all that, one of the best times to "try" shooting early is at
the very beginning of a bounce (emphasis on "beginning"). you know kitty's numbers are going nowhere but up so shooting 1 - 2 hours early could allow you to get back on schedule and/or get a jump on the
expected bounce. if your schedule is flexible or you need to make up time in order to get back on schedule, why sit around another hour or two and watch the numbers go higher? just remember to monitor closely and the next shot is due 12 hours later.
as always, there are exceptions to every rule. occasionally, an experienced member of the group will help guide a caregiver through a series of early shots with the intention of bringing a down a cat who has been experiencing high flat numbers for some time. i can't go into details here because these are customized plans based on the individual caregiver's requirements as well as the individual cat's needs.
important: extra care should always be taken when shooting early.
hope this helps...