First know that the body will stop making vitamin D before you reach overdose quantities. This means you should take vitamin D in the morning.m rather than the evening. Second know that vitamin D is fat soluble. So if you are losing weight, you can more easily overdose if you have high levels stored in your fat. Also know the body won’t absorb as much vitamin D if you don’t take it with fat.

This can make dosing tricky. You can be taking an amount that is safe right now, but then is too much later.

You can max out your body’s vitamin D production even on a cloudy day, though the sun’s angle of incidence effects production.

The body typically maxes production at something like 20k iu (pleae verify this number it has been a while since I learned it), so staying below this number should mostly safe.

The USDA has set its recommended daily allowance mostly to avoid rickets. It is largely considered too low a number for general well being.

I live in north western Washington, and previously used to combat seasonal affective disorder, with some pretty dark thoughts come february. Since I started taking 1k D3 some 20 years ago much of the seasonal mental health has gone away. I take 2k D3 consistently currently, and if I run out for more than a week my mood starts to deteriorate quickly. I still haven’t proved causation since there are likely reasons I’ve let myself run out of the supplement that long, but it is so consistent that I treat it as causal at this point. YMMV

Please do research above just asking a forum for dosing advice though. This is a well educated place, and I would very much trust it as a starting point, but there is a lot of good published content on the topic. Though, I admit google is so bad today, I might fail to find any of the content I referenced years ago… if you use chatgpt make sure to require references, and check them. I find that using multiple instances to review research references separately prevents some context based poisoning as well. And pointing out inconsistencies can be a good way to find nuance in a topic. Though sometimes LLM will just waffle, and the context may be done