Mixture control bottoms-out against the firewall?

I have a maintenance question.

I read somewhere that the mixture control should not bottom-out against
the firewall when full rich, as the pilot would not be able to tell if
the full-rich position was achieved, or if the knob just hit the firewall
stop.

However, I'm wondering if this is just a rule of thumb, or if there is
an actual FAR which dictates it, and which FAR that is?

Thanks,

--
Mark Hansen, PP-ASEL, Instrument Airplane
Cal Aggie Flying Farmers
Sacramento, CA