How would that work with a total duration not divisible by 0.4? In other words, suppose I get the duration to 1.5s. Since I didn't hit a 0.4 divisible breakpoint, a "normal" channel will still just get me 4 ticks. However, If I channel for .8s, I get three ticks (0.0,0.4,0.8) and still have 0.7s left.
In the above example 0.7s left meant I got 150% explosion, but it seems like in part that was because if I kept channeling I'd get two more ticks, where here if I keep channeling I'll only get 1 more - even though I have the same remaining duration.
So in that case, would I get the "extra" 50% base damage by ending early?
You won't get any extra damage or extra ticks for any extra duration, effectively rounding down to the nearest 0.4 seconds. The skill effectively increments of 34% increased duration.