Great thing is that Thumper is getting better #'s, even though it makes dosing tricky for her mama. Good thing is you have a weekend coming up and hopefully that will give you a little room to experiment and test. Often when a kitty is close to going OTJ (into remission), they will get low-ish pre shots but still in the diabetic range (normal is 40-120). This is when you would consider experimenting with micro doses. It depends on the kitty, but for some just a few days or weeks with a micro dose is enough support that allows the pancreas to heal. I can totally understand your feelings of being a "newbie," and it is a shift in thinking from original recommendations not to shoot a number under 200 to shooting a micro dose.
For tonight, you have a couple options depending on how Thumper tests. I don't think there is one right one. If Thumper is back in the 300's like yesterday, you could either shoot the same .5u or back off just a hair to .4u or .3u with the thought that, if you back off a hair, you'll have a slightly higher # in the morning that you're more comfortable shooting.
Like Lori said, if you have a number that is rising, you could consider a micro dose of .2u or .3u. Like you've been doing, when you get a low pre shot, you want to wait 15-20 min and test again (no feeding). Again, if the number is rising (you decide what range you're comfortable with and can adjust as needed...something like 180-200), you would consider a micro. If the number is holding steady, you could wait and test again. One reason you would consider a micro dose rather than skipping a shot is you want to head off your kitty's BG #'s going higher since the healing occurs in the lower #'s. For some kitties, diet change and enough time spent in the lower #'s with support from insulin allows the pancreas to heal and start working again on it's own. When this is happening, there can be "sputter" factor where the numbers can be a little jumpy.
You're doing good. You need to feel comfortable with what dose you give Thumper. That's why you ask questions. And who knows? Like Lori said, Thumper may surprise us all with a great # tonight. Keep us posted.