So I tried using ganache under my fondant and it was great but I am not a big semi-sweet fan. I notice the semi-sweet ratio is different from the white chocolate ratio. So what would the milk chocolate ratio be.

I used milk chocolate ganache under my dinosaur cake and did a 3:1 (this is the same ratio as white chocolate ganache).

Your Milk chocolate and white chocolate ratio's should be the same . They both have a higher fat content so it takes a little more chocolate for it to set up right
also remember to let it come to room temp before putting it in the fridge the moisture in the air binds to the fat and will cause it not to set right!! took me forever to get white chocolat ganache to set properly .
hope this helps

Thanks you!!!