From FSDeveloper Wiki
[edit] XML: Increment Variable w/Modulo Reset
The following script shows how to increment the custom (L:Variable,unit) value by 1 on each "click". The number preceeding the modulo operator (%) is the reset point (highcount + 1), at which point the variable will reset back to zero!
NOTE: this technique works for integers only. Fractional increment/decrement should use other methods.
0 + click = 1; 1 mod 5 = 1
1 + click = 2; 2 mod 5 = 2
2 + click = 3; 3 mod 5 = 3
3 + click = 4; 4 mod 5 = 4
4 + click = 5; 5 mod 5 = 0
<MouseRect>
<Cursor>Hand</Cursor>
<TooltipText>Window Left</TooltipText>
<CallbackCode>
(L:SwitchWindowLeft, enum) ++ 5 % (>L:SwitchWindowLeft, enum)
</CallbackCode>
</MouseRect>