Cascading will get you the most total levels granted.
But not the most total levels on Account A.

With:
A -- B -- C -- D -- E

Assuming you boosted two sets of 60's on B-E, we have:

A gets 50.00% of the levels gained by B = 60
A gets 25.00% of the levels gained by C = 30
A gets 12.50% of the levels gained by D = 15
A gets 06.25% of the levels gained by E = 07

B gets 50.00% of the levels gained by C = 60
B gets 25.00% of the levels gained by D = 30
B gets 12.50% of the levels gained by E = 15

C gets 50.00% of the levels gained by D = 60
C gets 25.00% of the levels gained by E = 30

D gets 50.00% of the levels gained by E = 60

That nets us:
112 granted levels on A
105 granted levels on B
090 granted levels on C
060 granted levels on D
367 granted levels in total.




If you go:
A -- B -- C
A -- D -- E

Again, assuming two sets of toons boosted to 60th:

A gets 50% of the levels gained by B = 60
A gets 50% of the levels gained by D = 60
A gets 25% of the levels gained by C = 30
A gets 25% of the levels gained by E = 30

B gets 50% of the levels gained by C = 60

D gets 50% of the levels gained by E = 60

180 granted levels on A
060 granted levels on B
060 granted levels on D
300 granted levels in total.