Judging the effectiveness of a charity on its overhead is tempting, but it's not necessarily the best approach.
First there's the fundraising: if for every dollar you invest in fundraising (organizing events, sending letters, etc.) you get three dollars, then you might rather invest $10K in fundraising and get $20K for your project than spend the $10K on your projects. So the question is not "what % the charity is spending on fundraising" but "what's the return on investment of the charity's fundraising?". Some charities do waste money on a lot of expensive fundraisers that just pay for themselves, while others got a much higher budget thanks to some clever use of fundraising.
Second there are the infrastructure. When you work in corrupt countries, if you don't have a good infrastructure, all your money will go in the pockets of corrupt officials. You need contacts, you need good relationships with officials and other powerful individuals, etc. And it's not always nice and clean. Some NGO have to deal with arms dealers, dictators, mafia bosses and other unsavory people to make sure that their help will get to the people who need it.
Of course there are charities that are completely inefficient. There are charities that pay more to their board of directors than they pay for their projects, but if you want to check how efficient a charity is, you need to look beyond the overhead.