How do I know what to give from a non-permanent income?

Seperated July 13, have been paying Maint since by joint agreement rather than through csa.

I worked out how much to give via online calculator, but ex now saying she entitled to part of my territorial army wage ( it’s a wage I get 1 months in arrears based on what training nights & weekends I do) friends in similar circumstances saying ex not entitled to part of this wage as it is non permanent income.

Nobody can give me answer on this and it’s very annoying not to know the answer.

If it is the case & I do add this wage to other income How do I work out how much to give per week as changes monthly dependant on what I do.