To best ensure your BX-1000 is measuring diameters at the correct depth, set the ball height (Height Over Ball dimension) with a PD-BX setting gage.
First, zero out the PD-BX gage on a gage pin or gage block stack up. Second, unlock the BX-1000 block assembly. Third, place the PD-BX over the contact point of the block. Then, simply adjust the ball height with a hex wrench until the PD-BX reads zero. Finally, lock the ball height into position. Make sure to repeat on the other block.