You need no money whatsoever on board other than gambling or gaming machines should you be inclined, as everything is charged to your 'card'. Your excursions are pre paid so other than the odd drinks & shopping money, so not a lot is needed there. If your going to pay gratuities in cash, then allow for that. As for being 'caught short' use the cash machines with your debit card ashore, as you would do in the UK, that way your not carrying bundles of unnecessary cash around.
Mmm difficult to say, how long's a piece of string? Reckon you do a rough calculation then add 10%. All purchases onboard are added to your account, drinks, purchases in shops, tips etc, so they will go onto your credit card at the end of the trip. Whilst off the ship travellers chqs are a good idea, plus a credit card and some cash. Buy some currency before you go as exchange rates onboard tend not to be as favourable. Have a great holiday.