I've done it two ways.

I prefer heel-lift mark time - heel comes up two-three inches as knee is bent forward for mark time.

However, when I've taught show style, they do a full foot lift to the top of the ankle - except when they do a high knee mark time (for effect) which lifts the arch of the foot to the side of the knee.

That being said, I've even seen show bands do a mark time with only one foot.