I'm currently making plum wine and I've always used 4 tsp. liquid tannin. It's good that way. (I want to switch to dry tannin as my liquid keeps going moldy.) My liquid and dry both are made by Spagnols. I emailed them and they said that they use 315 grams of dry in 1 liter of water to make their liquid. Sounds good but I have no idea, and can't find out, how many tsp. are in 315 g.
Personally i dont even mess with the tannin in plum wine, i like the fruit to shine. A lot of tannin powder is made from oak. So you could just add some oak to it if you are going for that sort of thing
You can also get some tannins from the skins, and seeds, but be careful how long you macerate. I added hot water to the skins and seeds, to try and pull out more sugar before i pressed them, and found out that it just pulled out a lot of tannin.