The most reliable way to dose is to get tested. Some people are extremely deficient and have a hard time storing it, so they require higher doses (this is the logic behind higher initial doses for a few weeks). They’re available online in the UK for £25-35, but I’ve seen some labs charge £90 or more, so it may be cost prohibitive where you are. Most people seem to agree 2000-3000IU is a safe long-term dose. If you live in Northern Europe, 5000IU would probably be okay. Making sure there’s enough vitamin A in your diet avoids potential toxicity.