So after what literally amounts to hours and hours of works, raffy and I seem to have worked our what we think is optimal usage for Force of Nature. The usage depends somewhat on your gear, but the basic idea is like this:
- You're at (or are about to reach) 3 charges.
- The encounter is going to end within ~20 seconds.
- At least one of your trinkets is up.
Now on the last line, typically what would happen is your Agility trinket procs and you spend 2 charges (remember, you're goal in between trinket procs is to keep from hitting 3 charges). Some amount of time later, your 3rd treant may finish recharging, at which point you should use it if the trinket proc is still up at that time. After the proc ends, you'll go back to keeping it from hitting max charges until you get another trinket proc.
Now there are two trinkets that present a special case in how you should use your charges (instead of just using them at any point during the trinket proc):
Rune of Re-Origination: Use them at the end of the proc (ideally about 1 second before the proc ends). This way the Rake benefits from the vastly increased mastery, and then after the proc fades they get your crit and haste back and they're auto attack does a bit more damage than if you had summoned it at the start of the proc.
Renataki's Soul Charm: Similar to Rune but for a different reason, you want to also use your charges at the end of this proc. This will get you an extremely hard hitting rake, which dwarfs the loss in auto attack damage that you would have otherwise had if you had used your charges earlier in the trinket proc.
In all of the trinket pairs that I tested, it was worth expending your charges when either
trinket procs, but conceivably if one of your trinkets greatly outshadows the other in item level that may be unadvisable. That would be something you would have to sim for yourself.
Finally, I should say that if you're dealing with procs that are longer than 15 seconds, it would be beneficial to wait until 15 seconds remains on the proc to allow for a small chance of multiple procs being active at the same time, but that is of course assuming you're not losing the benefit of some other buff because of it.
Edit: Just discovered something that makes me think this post isn't entirely accurate. Take with a grain of salt.