It's ok to replace back squat with front squat on madcow routine on light day??
Try it and see how it works out for you.
I would say yes if you keep the intensity on the light side. The offset in load is not THAT significant compared to the back squat, so if you go really heavy you will likely impede recovery and/or deadlift performance. But as Punisher says, give it a shot and see.
I know someone who's doing this with Texas Method right now. Just take 80% of your light back squat weight, otherwise it won't be a light day. So if the template tells you to squat 200 on Wednesday, front squat 160 instead.
and this seems a decent rule of thumb.