For a crash and decant scenario you want to get to full attenuation first. You could measure FG, but that would be a ton of hassle. I tend to just wait at least 72 hours and make sure the activity has died down. Also, your time to force crash is obviously dependent on the yeast's floc properties. With some yeasts, I've had to wait at least 48 hours more at 40F to get the yeast to floc to the point I felt comfortable decanting. Unfortunately this approach takes more time and planning foresight.
Originally Posted by LarryC
On brew day, do I just decant off the excess wert and then let the yeast get up to pitching temp and then dump them in the fermenter?
Yep, that's basically it. Just decant carefully like you would pouring a bottle conditioned beer. Do leave a little bit of liquid behind so that you minimize pouring off good yeast and can swirl all the yeast off the vessel.