How high off the ground for floating vanity?

That would be a personal choice and visually appealing. Also consider the plumbing connections within the cabinets.

To whatever is left over to achieve the 38 " height

How tall are you?? that is what matters

Comfort height, in measurements, is another term for counter height—usually about 36 inches in height, versus a more standard 30-inch or 32-inch bathroom vanity.Standard countertop height is 31 inches, but it's really a matter of personal preference. Taller adults might want a higher countertop, while shorter models work better for small children. Discussion on this https://www.houzz.com/discussions/818526/help-bathroom-vanity-height

You could realy make it to the hight you want so stand there and pretend your washing your hands and make a mark on the wall next to we’re you are for the top of the sink to go