First, you must know (not assume) the plate voltage to have any chance of telling whether the tube will even take it, let alone bias correctly at some particular current/dissipation.
And second, the 70% "rule of thumb" is questionable anyway. It's an upper limit, not a target.
The right current is above the point where crossover distortion occurs on a full-signal clean tone, and below the point where the tube ratings are exceeded when driven fully into distortion. Sometimes this is a small range, sometimes large. Often - with well-known amp/tube combinations - you can pick a figure which will be in the right range without having to measure anything else directly.
Another problem is that if you're using the common plug-in bias probes, the current you measure includes screen current (usually a few mA). Actually, this is a good thing in some ways as it reduces the real plate current slightly at any given measurement, hence making the setting a little more conservative - but most people don't seem to know this, let alone account for it. Either way, quoting bias figures to +/- 1mA, or anything like it, is inherently flawed.
