I'm not going to beat around the bush, change it to 12.5% to reflect the actual litter size. That's half of 25%, it's fair. 5% is just ridiculous.

At the end of all the items spent (buffalo scrotum, lion scrotum, and great tit at the very minimum, I use mine for projects so I also use gnawrocks and ochre saltlicks) you've spent far more than you'll ever realistically get back. My DUs rarely ever pop a DU cub, nevermind a nice or even moderately helpful one, especially if you're using them for projects which is all they're really good for, since they have no special lineart changes. The pass rate doesn't need to be so abysmally low for a mutation that's literal only use is for getting a few extra chances for a successful project cub and like...bragging rights, I guess. At least dwarves are cute to look at, DUs have nothing else going for them.