There are a number of equations used to estimate a horse’s body weight. Aside from the one you used, another common one is [(heart girth² x length)/300] + 50.

These equations were derived by measuring large numbers of horses and then weighing them with a scale. A statistical program was used to evaluate which measurements or combination of measurements most accurately estimates the actual weights.

The weight tapes you refer to are obviously using only girth circumference, which is why they give a different number than the equations. The difference can be negligible for some horses and significant for others, depending on the conformation of the horse.

From a statistical standpoint, adding in the body length measurement increases the accuracy of the reading, though statistically speaking, both are inaccurate. However, both weight tapes and equations are useful tools for determining changes in weight, especially if the same person measures the horse each time and the same measuring implement is used on every occasion.