When are Turnips Ready for Harvesting?

I'm on day 60 of growing my Purple Top turnips.  They're really tall and leafy already.  The packet of seeds says 40 days to maturity -- so we're already way past that.  The websites I've read say they are like carrots in that they will start poking their shoulders out of the ground when they are ready to pull.  One of mine is poking through, as you can see below.  However the rest are not.  So I'm thinking I should leave these in a bit longer.  Folks also say that when they get as big as a walnut or golf ball, is when you can pull.  So I don't think this guy is as big as a walnut -- so again, gonna let it be.

Turnip poking through the ground