Hi Mark,
Les Andrews' book "Model A Ford Mechanics Handbook Vol. I" says 18-11/16 +/- 1/2 inch to the CENTER of the front bumper to ground for a 1928 or 29 and 18-1/2 +/- 1/2 for a 1930 or 31. Measure with tires fully inflated to 35 psi. He does not give a similar measurement for the rear. Differences in tire size from different manufactures can change the heights.