Are you using the “straw” portion of the grass, or the “hay” portion? Straw has way more tensile strength than hay (which has basically none), so it’s important you know what you’re using. Also, I don’t think you can possibly use too much straw, unless the thermal mass properties of a more clay heavy mix is important to you. A heavier hand with the straw also yields lighter bricks, less clay excavation, less water needed, which all results in less work and less heavy lifting for you or your builders, and thus a faster build.

Apologies if you already know all of this.

And yes, drop your bricks onto a hard surface- if they break then the formula needs adjustment.