GGB As a working rule, rent is best kept in the high single digits to low-teens as a share of expected revenue; much above the low-teens puts permanent pressure on margin. It is a rule of thumb, not a law — your real numbers decide — but it is the single most useful filter on a site, because rent is the one major cost you cannot adjust after you sign.
From Restaurant Lease and Fit-Out: The Traps That Sink Operators — the full reading, with the method behind this answer.