I hope I'm not oversimplifying it, but the question seems like a straightforward math problem to me.
1. If you know the constant (Gross? Net?) you'd know your cost is (your chosen amount for that year) times 1.8%.
2. Take that same constant (for the new year, as I expect the figures could be different) and multiple it by 1.2%.
Subtract result 2 from result 1. My math shows that this third result, when divided by the first one, shows a gain of 33% (translated into 33% less loss) which passes the straight-face test when comparing 1.2% against 1.8%.