I understand your point, but I'd suggest it's only wrong because at the point the equation is defined Mathcad knows the input units but not the output units. If all units are known, either by defining the output units with the rest of the equation or (shock horror!) looking forward to where you want the result of the equation. Saying that, I realise that could cause its own issues, not least of which if you asked for the result of the equation twice, and selected different units.
I'm still undecided really. I do see the merit in using units and letting Mathcad take care of them, but at the same time I'd like the steps Mathcad is taking to be more obvious (perhaps a placeholder to indicate unit conversion - though I suppose arguably that too would be wrong!), even if they can't be explicit. It may in part be down to my weakness when it comes to maths - I'm much happier with a schematic or a spanner! I suppose one option would be a template with a footer carrying a short note explaining that the working shown was generated in Mathcad and that Mathcad is unit aware. As much as anything I'm quite conscious that showing the equation 'correctly' is identical to the equation quoted for Imperial units but different to that for SI units, so it would appear the wrong equation had been used when compared to the data sheet (I work pretty much exclusively in SI or derivatives).
Anyway I do at least feel I've got a reasonable understanding of what Mathcad is up to and the advantages and disadvantages of the different approaches, so I'll try and work out which approach to take. Thanks again!