Use a piece of string start from exactly from where your shifters are mounted on the bars and route the string like you would the blip cable to where you would want the blips. Then measure the string.

Not all 42 cm wide bars are the same in terms of reach, and not everyone mounts their shifters on the same place. Some bars allow for internal routing, and even if it's exterior, some people prefer to run their cables in front of, in back of, or below the bars. Then of course their is personal preference on where you want the blips mounted. I could have the exact same width bars as you and require 4-5 cm more or less cable than you based on my preferences.

I ordered 230mm blips for my 440mm bars, and they are the perfect length.

Edit: I double-checked my bar size, and they are 440mm, not 420mm as I had originally posted. I would still get the 230mm blips, just to be safe.