floor is rounding down - it acts like round() but when you have floor(1.9) it rounds down to 1 when round() rounds to 2, while ceil is the opposite it rounds always up, so ceil(1.1) becomes 2. useful with random().
back to your question, you can simply subtract 10 + something from HP you dont have to limit yourself to a value you can subtract a whole expression like 10 + variable1 * 2 - variable2 from HP.
or maybe i don't get the real issue here :(