I've not used DT so can't comment on those. Couple of hopefully- useful links:
Stumbled across this, which has ERD figure in it for Alesa Xplorer 26". It appears the same as the 700c figure in the same table. One must be wrong.
And in case you want to double check (advisable!)
Measuring Effective Rim Diameter
A crucial part of determining correct spoke length for wheel builds is an accurate "Effective Rim Diameter" (ERD), a concept that is reviewed in this article.www.parktool.com
Yes great idea. I will see how they work out.Measure in 3 places round the rim....add up and divide by 3. That way if the rims a bit oval or you were a little inaccurate, it will all compensate.