I did the exact same thing as Easy, with the exact same stupid post cap limit.

Essentially, it comes down to

~40*(GCD - s) > 67.2*s

Where s is the breakpoint at which its better to switch, aka, if the DoT is going to expire in < s seconds, delay GCD to refresh, otherwise, use GCD then refresh after that.

At close to 2.5 GCD, the breakpoint is ~0.93 seconds. Which means, if you can no longer see the number on the DoT debuff, delay until it disappears entirely then refresh. Otherwise, HS or other filler instead.

At closer to 2.0 GCD (aka monks), the breakpoint is ~0.7 seconds, which means its almost never worth it to delay GCD