I saw
@Chris & China (GA) advising you on FB. I don't want to conflict with anything she may say, so please take what she says over my advice if there's a conflict.
It is not you being new to testing! The number is the number, although I'll do another test or two if I think I got a funny number/bad strip (like I would have double checked the 203 you just got).
If that 203 is right, he's likely bouncing from that 90.
Does he have a history of ketones or DKA? You'll also want to get set up to test for
ketones (click for link) at home when you get a chance. If he does, then we do not want to skip the shot.
If he does have a history of those, and you have plenty of test strips, a hypo kit and are comfortable/successful with home testing, I would give 1.75U to be safe (but keep reading next paragraph before you do!), although he technically did not meet the requirement for a reduction. But without more data I'd rather err on the side of safety
If he has a history but you're low on strips/struggling with testing/no hypo kit, I'd give 0.5-1U, whichever you're comfortable with
No history, I'd skip the shot for tonight and resume in the morning so.youre not off schedule.
Edit: the numbers you're seeing are perfectly safe, by the way. Depending on the dosing method we reduce below 90, or below 50. You really only need an emergency vet if for whatever reason he's very low, showing hypo symptoms, and/or you can't get him up with high carbs, honey, etc. Which is very very rare if you follow the guidelines here.