You are being much too generous. When you are attacking you want to take down the enemy now, not in 3 rounds. If you are regularly taking 3 rounds to down normal opponents then your party is doing something wrong.
On going damage of the same type doesn't stack either. So if you hit the same target a second time often you won't be getting full value.
Well, yeah. That's why I put all those disclaimers in my post warning of those exact downsides.

1.5 is probably a good baseline as an easy way to eyeball how much you'll get out of it. It really depends a lot on style of play, and how much you'll adjust your tactics to compensate. Do you have a character who is likely to be in a position to put the ongoing damage on non-primary targets, while still helping to focus fire on the primary enemy? That's the way you are likely to get the most of it - and as it happens, something the monk can actually be quite decent at, if they plan their actions accordingly.
On the other hand, if you are expecting to just run up against one foe and focus on them, you are much more likely to lose out on some of the benefit. So if you expect that to be a likely approach, there is no real reason to take the feat in the first place.