Let's say I have a happy budew, will it evolve in the middle of level 27, or at level 28.
It will evolve at level 28.
hope this helps ;D i didn't do so well on this answer cuz im drowzy and stuff… :P
You need to put your Budew in battles and don't let it faint often to increase it's happiness, and you'll also be increasing it's level for future battles so you can just keep on training Budew and it will faint less often increasing it's hapiness more and more until you evolve it.
I'd say 29-31