This is a pretty basic feature development-wise which I think could in the long term save a fair bit of time for users. I often find myself changing the font size and then having to go and change the line height to match this again.

It would be great to either have a 'link' button between the two fields which engages the lock, or to have the ability to set the line height to 'Auto' whereby it simply matches the value of the font size.