Must Have At Least := xxx evaluates total number of ships/units
per empire. So to realize mass fighter production do something like this:
Entry 10 Type := Fighter
Entry 10 Planet Per Item := 50
Entry 10 Must Have At Least := 100
...
Entry 20 Type := Fighter
Entry 20 Planet Per Item := 20
Entry 20 Must Have At Least := 300
...
Entry 30 Type := Fighter
Entry 30 Planet Per Item := 10
Entry 30 Must Have At Least := 700
...
Entry 40 Type := Fighter
Entry 40 Planet Per Item := 3
Entry 40 Must Have At Least := 1500
and so on
These numbers are approximate, modify them according to your liking.
Don't use Entry XX Must Have At Least := 5000 from start, because AI will to get caught in an endless fighter loop and won't build anything more.
Such step-by-step approach will make your construction_vehicles file bigger and require a quite a lot of planning and testing, though.
PS: this is just a suggestion, it not tested by me, so use it at your own risk