If you want to count downwards do the following:

Say the max value is 10.

You want to decrement the number 6 with 1 which will give you 5.

Substract the 6 from the max number.

10 - 6 = 4

Add the value you want it to decrement (1).

4 + 1 = 5

Substract the result from the max number

10 - 5 = 5

You want to decrement the number 6 with 5 which will give you 1.

What to do:

10 - 6 = 4

4 + 5 = 9

10 - 9 = 1