well, to summarize in pseudo: Base Value * (1+((sum of percent modifiers)/100)) * multModifier1 * multModifier2 ... etc
modifyPercent adds a multiple of the base (so -100 would subtract the entire base)
modifyMult multiplies the total value.
the tip mentioned is in vanilla's tips.json, and is a pretty smart way of handling the hull thing.
the code change itself would be merely involve a lil math, taking w/e you were using for percent, adding 100, dividing by 100, done. if it's -20, it becomes 0.8. and of course swap modifyPercent for modifyMult. any further adjustments are after that shift.
for my mod, I mostly use modifyMult...strong bonuses, but also strong penalties (that also largely lack the likelihood of totally bricking something)
and yes I understand, holidays and the like. busybusy! heh. try not to get any pine needles (fake or otherwise) stuck in your socks...