Brisket boards 75mm high are adequate to position a cow. There is no benefit in having it higher, but it will make life less comfortable for cows.
The normal sequence of movements when a cow rises is shown below.
Picture 6 shows how the cows foreleg will be placed beyond the brisket board as the cow rises. The higher we make the brisket board, the more difficult it is for the cow to do this.
In the lying position, it is very common for a cow to extend her foreleg directly in front. A brisket board should not be so high that it restricts this natural movement.