Hi Ozzie
The intention of the procedure is "to confirm the accuracy of the alitimeter (in use)" as most ILS's are bringing you to within 200ft of the ground, I would suppose that this is a desirable thing.
Yes unexplained discrepancy is a bit vauge. It could be any discrepancy up to but not outside the IFR altimeter serviability of 60ft. In cases of two Altimeters it could also be 40 ft either side ECT.
It is really up to you to decide what is UNEXPLAINED.
As for the "OFF SLOPE" bit, I could get all stuffy and say, you should not be off the slope, but I would be bieng very hypocritical, I suppose.
One operator I worked for had a description in thier ops manual to assist in making the decision. I am going from memory so take this information as not neccisearily correct!!
1 dot = 20 ft at 1 nm
Therefore if your check Ht distance is 5.4DME 1 dot = 108 ft
If I can find the accurate reference I will post it, BUT once again this info may not be accurate.
richo