Ideally you will:

Boost with A.
Keeper B --linked to-- Disposable D
Keeper C --linked to-- Disposable E

You boost four toons on BCDE to 60th.
D and E, grant 30 levels to new toons on B and C.

You boost four more toons on BCDE to 60th.
D and E, grant 30 more levels to the toons on B and C.

For every two sets of four toons you boost.
The B/C accounts each get one free 60th.