The best times to engage head are when you have Shoulder Tackle off cooldown. You don't want to waste time running between the head and tail. Here's what I usually do:

a) At the start, before sahagin duo spawns. Position about halfway down the ship and move in as Levi lurches forward to engage quickly without getting squashed.
b) Before mage sahagin spawns. This applies only if you did not phase skip.
c) After first spumes are defeated. You'll want to save shoulder tackle to navigate spumes beforehand.
d) After first SP. There is a long period before sahagin duo spawns.
e) After second plumes are defeated. You'll want to save shoulder tackle to navigate spumes beforehand.
f) Depending on where second mage sahagin pops, you can engage head first immediately after defeating it.
g) After the last set of spinning dives. If Levi is low enough, use shoulder tackle to abort the sliding mechanic and engage the head to finish it off with LB1 and a couple flank attacks.