Yeah, I usually do 3-4 lbs fruit per gallon, but for something as subtle as star fruit, I could totally see doing 5 lbs. As you point out, you can always dilute later. I also usually do 1-3 lbs of sugar per gallon, depending on a) how boozy I want it and b) how much residual sugar I might want leftover. I feel like star fruit would do nicely as a dry wine, so maybe go for less sugar? At least to start? Adding some other juice (grape or apple) would help give the wine some body -- I'd go for white grape juice rather than red for fear of totally swamping the star fruit flavor. Just my 2-cents! Keep us posted. I'd love to hear how this one turns out.