HealthBoost causes the health charging to get . . . off
At the highest HealthBoost levels, the health charging doesn't line up with the Vanilla invulnerability.
In vanilla you get (20 ticks) after the Wither has reached full health before the explosion.
The invulnerability is 220 ticks.
The wither spawns with 1/3 of it's base health.
The Wither's vanilla base hp is 300.
The Wither increases 10 hp every 10th tick.
The current algorithm does not quite work:
((baseHealth * (2/3)) - 200) / 200
hp/1 tick
It gets to full health too soon.