My cat loves to eat grass. When it's too cold for him to go out, I'll grow some catgrass for him. This past spring I did a little experiment to see about optimizing what I grow for him. I took a mix of 1/3 peat moss, 1/3 vermiculite, and 1/3 compost (which if you do Square Foot Gardening you'll recognize as Mel's Mix) and a mix of 1/2 biochar and 1/2 compost and combined them in equal parts. I took out one 3" round pot's worth of that mix, which I will call a 1/2 biochar-compost mix, and to the remainder I added an amount of Mel's Mix equal to what I had left, giving me a 1/4 biochar-compost mix. I repeated this process down to 1/128 biochar-compost mix. I then planted them all with 30 catgrass seeds, set them out on my patio table, and waited a couple weeks until it was time to harvest. At that point there was visibly more catgrass in the containers in the 1/8 and 1/16 biochar-compost mixes than the ones with more or less.
The point of this wasn't to get quantifiable results but to have an idea of what range the biochar was most effective in. If I repeat this experiment I will likely do more than one pot of each, and actually measure my results, but I will probably only try between 1% and 10% biochar.