Make the system simpler - a flat daily rate for each specific "theatre"
ISTR that the 'world's favourite airline' do (or did) something similar - an acquaintance of mine used to get 15 days 'rates' (c. 50 GBP/day) paid into her bank account every month, irrespective of whether she was working for 20+ days each month, or just 3-4. Swings and roundabouts - some months she was quids in, others not, but overall she was financially better off at the end of each year and the whole system was simple to administer.
Obviously, you can't draw parallels between airlines doing repetitive routes day in, day out and people stuck in the sandpit for months at a time, but there must be some merit in looking at a similar system?