dpm goes to zero because 10 divided by anything greater than 10 is zero (in integer maths). Even if the multiplication happens first you get the problem with denominator greater than 30.
I am assuming that the key presses will be at least 100ms if they are being done manually, so this calculation will probably never work as you expect.
I would consider combining the rpm & dpm calculations into one and use brackets () to control the flow of calculation so that you do not lose significant digits in the integer maths.