I don't recall ever learning one, sjm. You don't like saying (or writing), for ex., three twenty-sixths? That doesn't particularly bother me. If it's a real oddity like that, I just call it a thirteenth or whatever's suitable. In this particular instance I'd tell myself, "I'm over a thirteenth done".
I find it MUCH easier to get through unpleasant things if there is a specific end time I can count down to: five more weeks in the cast--only four now; six more days and this horrible test will be over, for better or for worse; that kind of thing. It really unnerved me, not being able to count down to the end of labor!