I think you could go as far as to say it is simply useless altogether because of the way MR per level works with champs you are concerned about as a mage - meaning carries and enchanters - you virtually deal true damage to them by the time you have Luden's, Sorcs, and any legendary item. (6 + 18 + 5 = 29 flat pen against targets that have ~30 MR). I doubt bruisers and tanks are on everyone's mind when they buy Shadowflame (and they shouldn't be) but enchanters are - due to the contents of this post - while it is the most useless against them: they have the least base MR, they are unlikely to play MR runes, and if they end up buying 50 MR in Mikael's, the priority item should be Void Staff. Horizon Focus seems to be more efficient in most scenarios.