First, what the percentage applies to. Net losses over a period is the common basis: everything you staked minus everything you won, summed across the week or the month, with the percentage applied to the shortfall. The period length matters more than people expect. On a weekly calculation, a good Saturday can cancel out a bad Tuesday and leave you with nothing, whereas a daily calculation pays on each losing day separately. Shorter periods are worth more to you, and longer periods are worth more to the operator.
Check what counts as a loss as well. Some schemes exclude live dealer games, some exclude anything played with bonus funds, and some count only losses on specific studios. A 15 per cent rate applied to a narrow slice of your play is worth less than 5 per cent applied to all of it. There is usually also a maximum cashback per period, and where that cap exists it is the ceiling on the whole offer regardless of the percentage.
Second, and more important, whether the payment carries a wagering requirement. Cashback paid as withdrawable cash is worth its face value. Cashback paid as a bonus with, say, 20x wagering attached is a different product entirely: to see £50 of cashback you would have to put £1,000 of turnover through the games first, and that turnover has its own expected cost. At a 3 per cent house edge, £1,000 of turnover costs about £30 on average, so most of the cashback is bought back before it can be withdrawn.
So the question to ask is short. What percentage, on what basis, over what period, capped at what, and paid as cash or as bonus. Five answers and you know what the offer is worth. Put them into the bonus calculator if the wagering version is the only one available.