So I have a Saucer Magnolia im going to get kabudachi with cause it has 5 trunks, 3 of them were grafted on while it was in the nursery.

The tree's tallest branch is about 4.5 feet in height, the rest of them are about 3-3.5 feet.

I planned on doing a hack this winter once I see if the air layer has taken, if not ill save that branch and try again (its a diagonal slit style instead of ring).

But My question is, should I just hack the top off this year and then begin reducing roots next year? Or should I prune some roots this year if I take some off the top to balance?

I was advised by a friend whom is also an amateur at bonsai as well to take no more then 1/3 of the top off the tree in its first hack, and to leave the roots alone.

Anyone with prior knowledge / experience know?