I use that rackers calculator everytime and I fully trust it. It matches up perfectly when I dough in. After mashing for an hour, and doing some recirculation, everything compacts a little so you could maybe sneak a couple pounds in.
Maybe you could try some recirculating in the beginning, then top off with your extra grain at the 30 minute mark? When you add the grain, you might have to continue recirculating to make sure the new grains' starches/sugars get mixed in.