If I remember correctly you should be able to show "target polar boat speed" in knots and "polar performance" as a percentage of the target polar speed obtained but I don't think the H5 system displays VMG as knots.
I recall it has a VMG function in knots, both "VMG to wind" and "VMG to waypoint".
Consider the other options. I have also chosen to display the target polar boat speed and polar performance next to my boat speed, instead of VMG.
I also display target wind angle as well, especially helpful downwind as its quite variable to wind speed, e.g. moving between 100 and 170 degrees.