I usually start with 3 lbs honey/gallon & 3-6 lbs plums/gallon, though I think 4 lbs plums/gallon is fairly standard. The amount of fruit really just depends on how much fruit flavor you want in the end product. Do you want to showcase the fruit with minimal honey flavor or would you rather have a balance of fruit & honey? I usually try for a balance.
Also, you can add fruit to primary, or to secondary/tertiary or to both. Adding fruit to primary alone will give you more of a wine flavor, adding to secondary/tertiary will give you more of a fresh fruit flavor; I usually do both. I think doing so adds depth of flavor to a melomel.
I wash the plums & freeze them whole in large 2 gallon Ziploc bags, then thaw & simply squeeze them with my hands to extract the pits, it makes it so much easier; though watch out for the pointy end of the pits, they can be pretty sharp. Also, the freeze/thaw helps to break down the cell walls, which means more juice & thereby, more fruit flavor. I also always use pectic enzyme with the fruit, it helps to break down pectin & results in a clearer product, faster.
Hope that helps. Regards, GF.