In Admin > Fleet you can input initial value and the date when the system should start checking it from.
When you start inputting APU hours in JL you have to input it as a cummulative value, not as hours per single flight. With APU, Leon just stores the latest value you provide here, which should be updated every time you have the new one (also as cummulative).
If APU Hours input in Journey Log are lower than the value from previous JL or the minimum value indicated in Admin > Fleet, Leon will show the warning: APU hours too low. APU has been entered on dd-mm-yyyy as hh:mm .