milk has many qualities (lactose fat etc) that is beneficial to the dough. if you are using a cup of water or a 16oz of water or whatever your measuring process is, try using 5-10% milk to start and go from there. use whole milk, or even better, milk that hasn't been homogenized. milk is only around 80% water so if you are using a large amount of milk, using a few % more will/may be needed to get the same feel.
i've used milk. it works. but it isn't always a desired taste depending on recipe. made some 24h ny style dough with
100% high gluten flour
if i were to use milk, i would use around 58-60% total milk/water, assuming i am using 10% milk. but this is the flour and my desired stretching that dictates it
15% KAWWW (king arthur white whole wheat) + 85% HG flour
52% water, 10% milk
would be a typical milk-based dough that i would ballpark from
in a nutshell, 1 cup of water minus 2T + 2T milk would land you close to a start.