I use it at $1/day per audience or “thing” I’m trying to do. Some of the audiences are really small, like retargeting ones, so as silly as it seems you may not even have $1/day worth of action and you waste it. Website has 30 pages of services, I won’t be afraid to take the whole 30 pages and do and audience retarget for each page at $1/day ($30/day total) and then show ads specifically retarget of each page. Maybe the one page for that really weird device doesn’t get much action, but it will spend 20 cents a day for the one off ad it shows to the one or two people that visited the page. The rest of the money gets put into the pool and spent when the right folks for some of the other pages show up. It’s not just for testing