As Shawn already said the approach / flare depend on a lot of variables, one of them being wind speed and -direction. E.g. with a 20kts tailwind you would maintain a 20kts ground speed by having 0kts airspeed. But you could also attain the same groundspeed with a 30kts headwind and 50kts airspeed - you'd be much safer in what you're doing.
I'm not exactly sure if I'm on the same page with you but with the figure of 10ft /sec (or 600ft / min) and 20kts speed you'd set yourself up for trouble - unless you got a pretty stiff headwind blowing.
I think overall you never want to get into a position where you have so much momentum that it becomes the determining factor whether or not you commit to a landing: in my book that sounds like a loss of positive aircraft control. Normally you gradually bleed of airspeed while you decrease altitude and what started at 60kts (airspeed!) / 300ft altitude on your final approach should ideally terminate at 0kts / 0ft altitude on the spot that you planned to land on. Throughout the approach you can vary any variable at any time.
If you flare to bleed of momentum (e.g. to terminate an autorotation or Quick Stops) you'd have a much higher airspeed to begin with (where your descent rate does not matter as much) and you could either flare gradually for as long as you want over a long distance or commence a very harsh flare to shorten the distance required. All depending on a multitude of factors.