The working formula
Take the company’s haram or interest income, divide by total income, multiply by the dividend (or realised gain, on the stricter view) you received. Give that amount to charity without intending zakat. The purification calculator does this in sterling.
If you cannot find interest income, some screens publish a purification ratio per fund. Use that. If you cannot find anything, you should not be holding the name and calling it screened.
Gains versus dividends
AAOIFI-oriented practice often focuses on dividends, because the share price already embeds retained earnings. Some scholars also purify a portion of capital gains. If you follow the stricter view, apply the same ratio to realised gains. Unrealised gains are not cash — do not invent a liability to feel pious.