Not a company, but I think direct scaling based on time is about as fair as you can get. An employee that works 4 days is likely to produce more than 80% of th

Not a company, but I think direct scaling based on time is about as fair as you can get. An employee that works 4 days is likely to produce more than 80% of the output of someone who works 5 days, all else being equal, so even though fixed costs may not scale down, payment per "unit work" should remain constant. This may not hold if you went down to, say, 1 day per week.