I do about 5000 to 25000 a day along with K2-MK7 and magnesium. There were times in the past when I used 160,000 IU's per day for a few months to decalcify myself without issue but I built up to that very slowly whilst also taking K2-MK7 .8 to 1.2mg day. Now that I am much older I am more sensitive to larger amounts but at worse I briefly just get a little pain here or there. One factor is body fat which absorbs a good deal of D3 and that makes calculating measuring anything anyones best guess as it gets mopped up.
What I am more interested in measuring currently is potassium for prolonged fasting but I might just cheat and potassium-water-fast. Trying to avoid a lot of grey market equipment taking up space.