Yea I know what you mean, I keep track of all my litters on a large calender in the rat room. I've been debating getting a grease pencil to hang by the tubs so I can scribble on the tub when I see a litter, then write the date of weaning 3 weeks later.
As far as I can tell, they are ready to wean when their eyes have been open fully for a week. That is generally around 21-25 days, and the mothers will have their new pinks already. LOL, thats probably no more help!