When the starter is very active (doubling or coming close to it) in 6-8 hours after feeding at room temp (~75F), itís good to go.
You know Iím not a big fan of refrigerated fermentation for SD, that being said, for 62.5%HR and your ď1 hour bulk, two hours in balls then overnight in my cooler. The next day 4 hours to dough temperature of 65 degreesĒ fermentation routine, I think I would start with starter equal to 12% of the weight of flour. I could see it being as low as 10% or as high as 15%. You will probably need to pull out some of your formula water to accommodate it as Iím guessing your starter is wetter than 62.5%.
I would highly encourage you to do some small scale test batches Ė maybe 10%, 12%, and 15% starter and then adjust as necessary. There is just no telling how your starter is going to behave until you try it.
Hopefully Norma or someone else who has done more work with cold fermenting SD will chime in.