This is a useful starting point, and addresses how the mana boost affects required Troops to hit a target number of tiles:
In Grimm’s case, you can shave him from 10 tiles to 9 tiles with his Talent Grid mana boost and a Level 17 Mana Troop, instead of needing a Level 23 Mana Troop.
Personally, I’m stopping at +18 on all Heroes for now. I don’t see the last two nodes as being worth the cost at the moment, and prefer to put those 110 Emblems toward the first nearly 5 nodes on another 4* Hero.