Just noticed that there is an MS Registry setting, Computer\HKEY_CURRENT_USER\SOFTWARE\Xara\HTMLFilte r\10.0\MultiWidthBreakpoint, that is defaulted to 50.
I changed this to 75 (% implied) and created a 1280px wide Main and a 720px wide Variant.
The application still reports the Breakpoint to be 1000px, midway (50%) between the two.
Preview, however, shows the Breakpoint is now happening at 1140px, 75%, and so closer to Main.

Xara could expose this parameter in the User Interface and perform the right calculation for the changed Breakpoint.

This could allow you to realised a smoother jump at the Breakpoint whereas up to now you have to fine tune the font sizes to make the transition appear seamless.

Acorn