Have you considered bracketing it or is this a purposeful marketing point? If a graduate is offered 2 jobs at 39k and 50k, would they make more at the 39k job in the short term?
At 50k they'll take 42.5 gross, $46k (40 + 15%) is the inflection. Save special tax situations anything over $46k will get them more than having a <$40k gross wage.
Marginal rates (brackets) are a good idea. Right now, too complex for the capital markets around this product and we think (possibly) for the customer, so we're sticking to the flat rate.
You certainly know more about this than I do (running a company and all that), but the situation described in the GP strikes me as a likely real-world scenario, and a bad outcome for both you and the customer.