Arr pirates!! As pirates I bet ye know all there is about Rum! Well ye don't! :)
Rum production is actually a very good indicator which shows ye if y'arr doing good or not. Ye see if yer Rum production is for example +3000 then this means ye got room for 3000 more pirates!! That's huge! What that tells ye is that y'arr not managing yer Haven well. The only time this is not yer fault (or maybe it is) is when the enemy attacks and destroys yer army then this is how ye see all those Havens with no gold or lumber and 50,000+ Rum. It means they don't have an army.
So back to what I want to teach ye and this is that ye got to try and keep yer Rum production near zero! Actually, if y'arr doing really good then ye probably have a little bit negative Rum production.
And by the way all of the above is while you are constantly building and upgrading yer Rum distilleries!!
So what's the logic ye say?
The logic or the purpose is to have yer Haven absolutely full of pirates / army up to the maximum limit. Have no barrel of Rum wasted at all! Otherwise ye be too fat for yer size. However, if ye keep on training troops and upgrading yer Rum distilleries and have a Rum production near to zero then ye be very strong for yer size and less people will want to fight with ye!
And don't be afraid to be having negative Rum production either. 100 negative means ye just gotta have 2,400 Rum extra to pay fer the day, which is super easy. Feel free to have 20,000+ Rum on ye and go all the negative ye want - Rum is the easiest resource to plunder and if ye be in a good Brotherhood people be giving ye Rum fer free as much as ye want.