Retina Sports finder for parallax correction?

[mh2000]

I just got a Retina sports finder for my IIa... can't resist classic accessories that seem useful and only cost a few dollars... it's a spiffy little gadget.

Anyway, I was wondering if anyone else is using one... and if so, how precisely does the framing work out? How good is the parallax correction?

In general, I like sports finders.

thanks!