This is a back of the envelope kind of calculation to provide a first-level approximation for the required break-even sales, given the GP% and fixed cost.

I say for start-ups, because many of my clients are early-stage companies and the required assumptions are a good fit.  For early-stage companies that provide software products or are a fabless semiconductor company, the primary fixed cost is people.  I have experience as the VP Sales and Marketing (and officer of the company) at a couple of pre-IPO start-ups, and as a member of the senior management team we used the same calculations as a simple sanity check for several variables.

Any college text on Finance or Accounting will have a formula for break-even analysis in terms of fixed and variable cost, i.e. S=F/(1-V/P) ; where S=beak-even revenue, F=fixed cost, V=variable cost and P=selling price.  Our assumptions are that all cost are fixed and that we use the average GP% for all products.  We would rather express the formula in terms found in an Income Statement and then we could write S=OpEx/(1-COGS/Sales), but we note that 1-COGS/Sales can be replaced by GP%, so now we have S=OpEx/GP%.

Since we assumed that all our costs are fixed and primarily consist of salaries, if we use the average monthly salary x the number of employees x burden rate x 12 - it would equal our OpEx.
EXAMPLE 1:
# of employees = 45.010288 (we need to use six digits)
Average Salary = $6,000/mo
Burden = 1.08 (accounts for fully loaded employee w/benefits, etc)
Average GP% = 35

Therefore, S = (45.010288 x $6,000 x 12 x 1.08)/0.35 = $3.5M/0.35 = $10M/yr


EXAMPLE 2:
The OpEx in EXAMPLE 1 was $3.5M/yr (if we divide by 12 it would give us a monthly burn-rate of $291,666.67), so another quick calculation for the break-even revenue could be:
S = ($Monthly Burn-rate x 12)/GP%

Once again, S = $291,666.67 x 12/ 0.35 = $10M/yr.


EXAMPLE 3:
How may employees could we support if our revenue was $10M/yr and our GP was 35%?

# of employees= Rev x GP% / monthly salary x burden rate x 12
# of employees= $10M x 35%/ $6,000/ mo x 1.08 x 12

So finally, # of employees=$3.50M/77,760 = 45.010288

I have created a break-even calculator in Excel that allows you to enter the variables (including EBIT for profitable companies) and quickly solve for headcount or break-even revenue.  You can request it by clicking this link -> Please Send Me the Break-Even Calculator.