OzEx, old chap, how you doing?
I'm no techie either, but while we wait for someone who is - I find it sufficient to understand that the DME equipment merely measures the time for the DME interrogation and response, and then does speed x time calculations to work out distance. Relatively straightforward, then, to "frig" ("introduce a correction factor to"!) the calculation the equipment does, in order to get the DME to read zero at THR.