I realize we talked in slack, but I figured I'd answer here too publicly.
Yes, the longer you leave it open the more sample data it collects. It watches your steempower balance fluctuate and estimates based on that slow increase. I think currently it's limited to 500 data points (which is like once every 63 seconds? or something?). But realistically if you leave it open for 10-20 minutes it is probably as accurate as it's going to get.
It's still an experimental feature and I make no promises that it's accurate. I'd love to know how to improve it though if the community has ideas!