I cant really answer your question re timing, however it has been my experience that once ringworm is in the barn it is very hard to get rid of. So after this group of calves you can bet on having some more each year.
Actually, it just occured to me that the times when I bought in a steer with very bad ringworm, and I treated it straight away, the other calves didnt get it. But when I had it in the shed from a previous group of calves, I always had a couple break out. So your calf might be alright but as you say I wouldnt move it into another pen now. Also, try to avoid the one with ringworm having contact with the others, like when you tie them out of their pens and also contact through the pen rails etc.
I wont go through treatment cos that has been discussed here numerous times.
But in the mean time, use the same halters, brushes etc on the two animals and once you have sold/slaughtered them, disinfect this equipment as well as the pen with bleach before using on another animal.