I've mostly worked in public-sector bureaucracies, not software companies, so my experiences won't line up exactly. In each case, the rule has been strictly pro-rata. I've generally felt that the company has had the good end of that deal, especially for the higher fractions (0.6/0.8 of full time): productivity seems to me to be non-linear, and trading off the end-of-the-week burnout for a little more payroll admin an…
1.0 time. 40h - 4h overhead = 36h. Pay = 1. Work-to-pay ratio is 36h:1p.
0.6 time. 24h - 4h overhead = 20h. Pay = 0.6. Work to pay ratio is 33h:1p. You'd have to suppose that the 0.6 time worker is at least 1.1x as productive per hour just to break even. If you provide benefits that don't scale with fractional work you need even more of a productivity edge to justify it.
The corollary of this is that you'd really better make sure you have little fixed overhead time for your workers if they are working fractional time.