The value of an amorous philtre sold at a tool vendor is 500 for 5 uses, so one would expect that each use is worth 100. In fact it goes like this: 4 uses is 420, 3 uses is 340, 2 uses is 260, 1 use is 180. So the formula works by taking the total price and subtracting a "cost" for each use, which ought to be 100 but is in fact 80.
Fix:
bump the "cost" of each use up to 100 at a tool vendor;
or divide the maximum value (500) by the number of uses (5) to get the cost of each use (100); or
do it the proper way and store the value of each use, not a maximum value, and value the total as use_value * number_of_uses.
Note: I haven't tried this, but the same problem may apply to other multiple-use items like powders.