Donk bets are usually from weak players at micro stakes. Strategy is COMPLETELY dependant on the villain. A classic fish example is that they donk with medium strength hands because they don't feel comfortable check / calling. Another example is a donk bet on for example a 2210r board that's usually a bluff because fish love to "trap" like Phil Helmuth and check raise. Note that these are only general tendezies i've noticed in soft games, it all depends on your villain.

There is no formula - but generally they are weaker. If fish donks its basically weakish - if regulars donk, that can have a lot of reasons and ranging and history becomes important

call your weak hands and raise your good hands