5lb plums and 2 lb of sugar to the gallon.
The purist will go for no added sugar for distilling.
I work with "fermenter gallons" not gallons of added water. Personally I use 5lb plum to the fermenter gallon (increased to 6lb if it includes victoria's) and later add 1lb sugar for each full gallon of wash in the fermenter.
It works out to only be a small amount of added sugar, sort of a compromise. If you have enough fruit, do different recipes to see which you prefer.
Leave plenty of headspace, plums foam up A LOT in the fermenter.

I freeze the fruit first then defrost it in the fermenter and add pectic enzyme. I only add 1/2 the total water to the fruit and make up a yeast starter in a demijon with 50:50 diluted wash from the fermenter. Starter is left for 24 hours then dumped into the fermenter.
I ferment the thick fruit pulp for 3 to 5 days, then add the rest of the water and the sugar and leave it till it has finished.