If your goal weight was 120 lbs, what would your +/- be?

Do you weigh daily? How big are your routine daily fluctuations in weight, from one day to the next, typically? I'm talking about weigh-ins under as standardized as possible conditions, like first thing in the morning, in the same state of (un)dress, after the bathroom, before food/drink. If you're female, adult, not in menopause yet, look at that question over a whole monthly cycle.

I'd suggest setting your upper and lower limits - if you want to use limits like that - at plus and minus just a bit more than your routine fluctuations. For example, if you often see 2 pounds up or down from day to day or over a menstrual cycle, set your range at 117 to 123. Then, if you go over/under for more than a day or two, adjust your habits in the needed direction. (I wouldn't do it based on one day, because sometimes one day is Just Weird. Use longer patterns.)

Truth in advertising: I'm sloppier than that, especially now (year 7 of maintaining). I tend to gain a little in Fall, lose a little in Spring, and have peaks after holidays or vacations, and that doesn't really bother me. If my same-size jeans fit comfortably, I'm not freaking out, though I do make it a bit of a point to adjust before I get there. I started out with a goal of 120 pounds (at 5'5"), and a view to handle the range as suggested above. Later, I decided 125 was going to work better for me, plus the more relaxed approach I'm describing in this paragraph. Right now, since it's fall, I'm up a few pounds. Meh.

Exactly what to do is very individual: Your preferences, personality, strengths, challenges, etc. - those all matter. It may make sense to start on the stricter end (as long as that doesn't make you over-anxious), and maybe relax later as you gain confidence.

You will figure it out. Trust yourself.