The ratio is the same regardless of how much doubler you have. You are trying to figure out how much of your stash to combine.
100mg / 18mg = 5.55555
So there will be 5.5 parts, one of which will be 100mg. That leaves 4.5 parts doubler but fractions will be a pain to work with so double the recipe to 11 parts.
2 parts 100mg + 9 parts doubler = 18.18mg
Math check:
200 total mg / 11 total parts = 18.1818…
ETA: If you want to have easier numbers to work with, do the math check on the whole numbers of either side of 5.5.
If you use 5 parts total (1 part 100mg + 4 parts doubler) you will get 20mg
If you use 6 parts total (1 part 100mg + 5 parts doubler) you will get 16.67mg
Both of those are probably close enough to fool your body.