I think there's a fairly open question about how those percentages are calculated. Oxfam is an outlier, with a very low stated percentage being used for overheads, but it'd be interesting to see a breakdown of how the remaining cash was used. For example - are staff costs in a target country (say, Haiti) considered admin, or is the UK base only counted?

Personally I would like a general rule of no more than 10% split between admin and fundraising, with 5% being considered the normal. 10% makes a nice Schilling fence.

Ideally I won't allow any budget for advertising, because that will just move money from one charity to another.

Alternatively I would only donate to the top of Give Well.