I can help you do the maths, but I don't know if your milk chocolate recipe will be a good one or not.
Your milk recipe: 2 parts powdered milk (assume whole milk powder [WMP] with 26% fat content) + 3 parts sugar + 5 parts nibs
= 20% WMP + 30% sugar + 50% nibs
Fat content approx = 26% x 20% + 50% x 52% = 31.2%
Chocolate grinders prefer to work with 36%+ fat content. Too low and it puts a lot of strain on the motor and may not be able to keep the wheels spinning (especially without any additional cocoa butter or lecithin).
Your current recipe = 70% nibs + 30% sugar = 654g
So, currently nibs = 458g (= 654 x 70%)
If nibs in your milk recipe = 50%, then total batch size for milk recipe = 458g / 50% = 916g
So your milk recipe is
WMP = 20% = 0.2 x 916g = 183g
Sugar = 30% = 0.3 x 916g = 275g
Nibs = 50% = 0.5 x 916g = 458g
Total batch size = 183g + 275g + 458g = 916g
In your current 70% dark chocolate you have
Sugar = 196g
Nibs = 458g
So, to create your milk chocolate recipe, you need to add
Sugar = 275g - 196g = add 79g of sugar
WMP = add 183g of WMP
And nibs are at the correct amount already
This will give you a new batch size of 654g + 79g + 183g = 916g
BUT, as I mentioned above, this isn't a recipe I would run through my grinder - I would be worried that the fat level is too low and might damage my machine.
If you look around this forum, there are plenty of milk recipes with a fat content closer to 36+%.
Eg.,
chocolatealchemy.com/milk-chocolate-45-cocoa/You can then use the same maths as above to adapt to that alternative milk chocolate recipe.