How about

estimate

revenue per customer

times

number of expected customers

should always higher than

variable cost

...

That might be the break even point for short term

...

When business starts

monitor number of customers

if it is lower than the figures you had calculated might be an indicator to decide to change anything to improve it or time to cut loss

...

If survived or established, the new break even will be re-calculated with those fix cost

BTW fix cost should be thought as stop lost amount ?