999 style master push rod adjustments?

[Slide Panda]

Since I'm already doing some work on the brakes  I figure I might as well get all the info I can.

I've got the 999 type masters on my Monster (thanks 'mash).  There's a push rod that connects the lever to the piston plunger.  It's threaded and can be adjusted. 

The question is, how do you set the length of that push rod properly?  I'm aware that if you go too far it restricts the fluid return, which is bad mmm kay.  Too little and you've got a lot of unnecessary free play. 

And is there an accurate way to gauge that free play?