f there is ILS on both ends of the runway, DME should be sited exactly midway between the two GP aerials.
While I've noted use of the word "should" in the above quote, this sounds like a country-specific policy. I've never heard of it in this part of the planet and, as far as I'm aware, none of our ILS approaches have ever been considered as "non-standard". For many years, we had a DME that wasn't sited very conveniently, but have always provided a DME/Altitude table - and continued this practice after solving the problem by installing DME at each GP.