Fro the Drakonix I think your assumption are wrong. It should be treated as a definate number of Leadership instead of leadership % since it affect only Red Dragons and not creatures in general.
You then get a definate +400 Leadership: -20% on 2000 --> 400
With Glory you know that you need 27 runes for 700 leadership, so every 100 leadership worth 3,86 runes
So the Drakonix value in runes is only 15,43, less then your 18,3
Then again are numbers compare!
Considering that this necklace cost 150k, it really doesn'T worth it!
Love the exercice!
I think, for all the items giving -% leadership requirements it should be base on the mean cost for that race leadership value. I think some race cast more then others for same level units. Or maybe evaluate the added damge link to that %leadership (having 10% more foreman per exemple). Will be tough but it seems that %leadership do tend to favor items giving that kind of effect in your results.
PS KongMysen does make a good point the 3 items giving runes afetr battles, the runes should be counted.