I am importing goods under an FOB contract. I know the overseas freight amount, but not the overseas insurance amount. Do I have to use the ten per cent uplift option?

No. The ten per cent option is not mandatory. If you do not want to use the ten per cent option (or you are not eligible to do so), and do not know the overseas insurance paid for the goods, you could calculate a soundly based estimate. Such an estimate may be based on a reputable commercial quote, industry averages, or several similar previous shipments for which the insurance amount is known.