you can't easily overdose pectic enzyme. I always add a lot to all my fruit wines. Adding more will give you a better, clearer plum wine with less sludge. I won 2nd place in a wine competition for dry fruit wines using drumroll......Italian prune plums vs ~15 other wines.
PS Italian prune plums make killer sherry if mixed with dried figs and sultana raisins, oaked, allowed to bake in an attic with enough alcohol so it won't ferment such as - demerara rum (best you can find), Grand Marnier, Drambuie or Metaxa. I like dried Turkish figs that I get from a Greek deli together with Turkish sultanas. California sultanas and dried figs are equally good. The Turkish Calimyrna figs are lighter than the California Mission figs but they both work. You can even add sliced over-ripe bananas and ferment with EC-1118 yeast. Trust me if you bake this wine in a hot attic after it stops fermenting with medium toast American oak you can get a spectacular wine that can age for 10 years plus that can develop a gorgeous creamy, caramel and nut flavour and smell. If I had Italian prune plums which I don't I would absolutely make this recipe. I've done it in the past and always got something really good from it.