I think that is a good plan, assuming you have an active fermentation going on right now.
If you cool everything this evening, some very health active cells will drop out and you can decant most of that liquid on saturday before you pitch. I would probably decant 3/4 of the liquid right as you take out of fridge on Saturday and let warm up 4 hours before you pitch, so plan accordingly.
I don't think there is any good way to tell how many cell you will be pitching this way, but you can sort of judge based on the cake your starter created today.
I would take off the stir plate and let set a little 30min and see how much yeast settles out or what the head looks like. If there seems to be very little activity or yeast production you might want to just let go until tomorrow evening, cool and decant on Saturday.
I wouldn't really stress much over it. It is more important you have good O2 in your wort and the wort and yeast slurry are the same temp when you pitch. Guys in our club are making a beer and as they are cooling the wort, they transfer a finished beer to a keg and then put their cooled wort right on on to that yeast cake in the same bucket the just transfered finished beer from.