Batter

I have a cistern excavation to calculate. The excavation has to go down to a level of -3ft below grade and I then have to place 12" of fill in the bottom of the pit. How do I establish a zero batter for the cut and fill. If I simply change the batters for the cut and fill to zero a red border pops up around the respective boxes, which I assume means there is an error.

Why is there no batter? - because firstly the Contractor will not get paid for over dig and we've agreed a rate on that basis. Secondly the ground is very stable, meaning that there's very little risk of collapse, hence the reason why the contractor will not get paid for over digging the size of the pit needed.


Thanks

Kevin