The Population screen already has a "next settler in", so it shouldn't too hard to also have one below it that shows "time to full" (i.e. time until you reach your population limit). I know you could theoretically work it out from the info already there, but it would be easier to show it. There is already room on that screen. This way you could work out if it is worth boosting any schools with the Lemon Tea, etc or the Town Hall. If you'll be Full in 3 hours, you can then quickly decide if it is worth using a 8-hour buff.