Thanks - we had discussions about this exact issue. We wanted something that wasn't a checkmark in the icon.

We played with another variation for quite some time but it didn't indicate anything related to tasks/completing something.

I really like what Apple has done with the icon for iOS "Activity" app that accompanies the Apple Watch. That the conveys a similar message to what we're doing.

(I do confess: I've tap-held our app icon from Springboard a few icons, which enables the app wiggle for rearranging apps)