For the larger forms I use a figure 8 and the smaller ones I was gifted a set of "Tolly Gauges" from Johnny Tolly down at SWAT. He designed them and they work great for small HFs. Of course I find it really easy to do consistent wall HFs if I pierce peek-a-boo holes in the side...all the way down... prior to achieving final thickness. Then I can see the actual thickness instead of measuring it. Thinning to a light source is also very helpful.