I'm not a math whiz; yours looks accurate to me. I just can't justify the amount of squares it would take to get the same results in my cities. An admitted hoarder, I also have veteran cities that have been doing tourneys, etc for years. While my inventory of spells is probably outrageous, just 10% of my EE spells alone would generate the same amount of mana (on demand) via Dragon Abbey:
280 completed provinces x L30AW power of 40 = 11,200 per spell
400 spells cast = 4,480,000 mana with zero decay*
This is in 20 squares. If you place a Maze of the Dark Matter, decay can be reduced to 2%, and with the mana also collected there, you could probably reach close to the same amount post decay, along with Enar's.
For me, that would be too much to coordinate mana supply/demand around scouting provinces. Much simpler to just sparkle my city whenever I need mana. Even for a player willing to do that coordination, they're still looking at using 58 squares to do it.
The perfect solution, of course, is to place all three of those AW's, lol! I remember when finishing S&D, seriously considering placing the Maze as well. But, space was at a premium, so 20 squares for Dragon Abbey was it. That may be different now. As demands increased on other decaying resources introduced in upper chapters as well as sentient goods and those factories, etc., I've needed AW's that improve production of those resources.
Don't care if I place the Maze, but sure would like to be able to justify placing Enar's (did I mention how pretty it is? lol!).
*zero decay: you use the spell when you need the mana, spending it as soon as you collect it.