Normally I write everything I've eaten down for the day, but today was so busy I got off track and thought I'd do just fine remembering. Apparently not. I just sat down and wrote everything down and did the math, and I'm stuck in the realm of "I'm either super close to my intake for the day or way off, what to do?"

According to my math (which is terrible) I have 258 calories left for the night. I'm supposed to have 1477 in a day to lose 4lbs in 2 weeks. Will being under my intake for one day be that bad, if I am wrong?

move on!
We all have days where we are over or under and this is just the realitiy. eat light if you can tonight and start fresh tomorrow. I always feel like days like that are reminders of why I still track my calories on a daily basis.

Dont starve yourself tonight trying to make up for it. Eat a healthy dinner and start fresh in the morning.

I normally keep 100-200 calories for a snack around 11pm because if I don't eat every 4ish hours (aside from when I'm asleep) my blood sugar gets wonky. So since I'm estimating 258 calories, would it be a good idea to only eat a 100calorie snack, or even less than 100 calories for the 11pm snack?

stick to your normal routine. Go on the lighter side of the normal range if you want, but if 100-200 calories has been working for you then stick with it.

Weight loss is not a mathematical equation. It has to work for life. If your blood sugar gets wonky it will be that much harder to be on plan tomorrow. Dont try to make up for what has already happened, just move forward.

Not sure if you zig-zag or not, but maybe you should just count today as a higher day (even though you are not that much over) and get right back on plan in the morning.