I've recently been looking over ROP:I to compile a bit of an index of demonic powers and I realised that there are no ranges, durations or targets listed with the powers. I am assuming that this is an edition thing because ROP:F has included these. Have I missed something? How do other SG's work out the ranges etc?
Well it's not, strictly speaking, an 'Edition thing' since all of the RoP books are for 5E but there have been some changes in the line format.
Specifically, the Divine (2005) and Infernal (2006) books don't include level calculations for their powers while Magic (2008) and Faerie (2009) do. I'm not entirely sure what the reasoning was behind this, but since demon stats in later Ars Magica books don't rectify the situation, I suspect the reason was that, since players don't play as angels or demons, the editor didn't see any need to include those particular stats. Please note, however, that prior to the release of those two books, the powers used by Magic and Faerie beings were likewise undefined in terms of Hermetic parameters.
Anyway, in answer to your other question. Generally speaking, I judge the RDT of most creatures' powers based on the written description of the power where possible. Where it's not, I tend to pick whatever seems appropriate. Is there a particular power that's giving you trouble?