As BMR says, you would be better to contract with a feedlot at that weight. The last 2 weeks in April and the first week in May is the best time to market grassers. 800 is too heavy for many grassers. The sweet spot for them is 680 to 725. In that range, you can pretty well bet on 200 lbs on grass, if not 250. I aim for my grasser buyers to hit just under 300 lbs of gain or so. That way, we both make good money.
The key is to hit them heavy with protein and little energy. Get the frame on them, a little bit of muscle, and keep the fat off of them.
For the last 10 years, we have consistently obtained a $1 or more (most cases $1.10 +) for our 680 to 725 lb grassers sold in the 3rd week of April. 80% of the time it is to the same local buyer, although a lot of the order buyers are bidding on them as well. Green, ready to gain cattle always will get a grasser buyer excited.
I only grass my fall calves to yearling. I don't want to grass spring yearlings because the risk of losing them to predation or sickness is much higher in the summer when then are out on pasture than when they are wintering at home. Grassing fall calves is enough work for me, along with everything else.