Dunno. In 47 years air-trafficking and flying I've never had a QNH off the sub-scale. QFE, yes, but not QNH. (It might be that there have been some cases recently West coast of Scotland or Ireland when it was Variable Force 11 Locally 12...).
Say the QNH is just off the sub-scale at 949 hPa, and elevation is 618ft / 21 hPa, then QFE is way off the scale at 928 hPa. Using the tables QNE is 2412ft ie. FL24-ish on touchdown. 'Sea level' is 618ft lower ie. FL18-ish. 3000ft altitude is therefore FL48-ish.
Probably best to have two brains checking the calcs...and extra guy ropes on the tower…and a Weather Standby…and a couple of nappies...