Define viewport height

I use viewport related units, like vh and vw, a lot! It would be cool if there was an option to define the viewport height either globally or - idealy - for each breakpoint. I also think this would help to solve some of the persisting bugs with previewing things like vh units. The viewport height setting that I'm suggesting could be visualy represented by a dotted line or not be visible at all and just be use for correct css calculations.