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On CRTs that have gone unused a long time my typical procedure on my B&K 466 is to do the typical check emission procedure at 6.3V and if it starts bad I let it sit on the tester in 6.3V emission check mode like that for as much as an hour. If still stone dead or very weak I'll bump the heater to 7 or 8 volts (depends on how strong of a nudge I feel it needs) and let it sit as much as another hour. If after that there is barely any emission reading above 0 I'll get a wooden dowel and start wacking the neck (hard, but not enough so to break it), as I wack it I try the rejuve settings one by one till I get improvement or failure.
If it stays stone dead 0 emission that usually is open cathode I deal with that by reflowing the solder on the base pins if it has a base like that, then recheck and do the reweld procedure in the tester's manual.
On my 466 I've found that the good bad scale is more geared toward color CRTs that need higher emission for a given brightness. Upper bad and "?" monochrome CRTs often are bright enough to use in normal room light if the chassis driving them is good and was engineered to give the tube plenty of HV...I prefer strong tubes, but in some cases taking "probably usable" is the best/only option. I'll often avoid doing a rejuv on an upper bad monochrome CRT (unless I know it is too dim to use) as I feel the possible improvement is not worth the risk of burning it out on a rejuve.
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