100% is never going to happen , if you can get 10 percent or less overhead that's great.20 percent is the minimum for me.Anything over that and it's either not efficiently run or has a truly difficult mission which eats up expenses.
To get 100 percent, there would have to be all unpaid volunteers working in donated space and getting their utilities, supplies, vetting the people/ organizations they're supporting (to prevent fraud) all paid for by others.
My go-to charity for things like this is the Salvation Army.Their total overhead including fundraising expenses is usually around 15 percent meaning 85 cents on the dollar goes directly to those they serve.They have regional offices , called missions and so any money sent to the Houston area office would go there.I sent some money to them after Katrina , it was their mid South mission in Mississippi and I received a thank you letter and much to my liking, never received any more mailings imploring me to give.
https://give.salvationarmyusa.org/si...donation=form1
BTW, I've already heard people are setting up Go Fund Me pages claiming to be flood victims and a few have already been revealed to be 100 % frauds.