That's a terrible rule of thumb, and there are tons of 10:1 engines running around on 93-94 pump gas w/o any issues. There is no "rule of thumb" because the amount of
variables that affect suitability are nearly endless. From head and block materials,design,cooling system quality & effectiveness, spark plug location & cooling, quench vs.
no quench vs squish vs the terrible zone in between, piston design, intake system fuel distribution variation,....................it doesn't end there but You get the idea.
