[stanleydrew]

What if the model were changed to

    M + T*Q = H

so that time and quality of the time are tightly coupled. Unpaid overtime that has a high quality might then still pay off (although this is also true in the case where T and Q are additive, just tougher to achieve).

[stephengillie]

Since Q is already a component of T, this would cause both to decline even faster. The more you value your off-time, and the less you have, the less happier you'll be.

What you're looking for is W, the happiness you get from your work. To modify the equation again:

  M + T*Q + W = H