How to thin half and half to substitute for milk?
Take out 4 tbsp butter and add the 1.75 c half and half.  That's what I'd do anyway.  It's about the same amount of fat and adds a bit more liquid back in.  Keep in mind that butter changes consistency depending on when it is put in or how it is applied, so it's possible this won't work so well if you have to do some whipping or something.
Do not add water.  You'll lose out on flavor.