If you add a mash cap with a recirc manifold, I would say that you could go down to 20 ppm.
Do you have any cheap suggestions for a mash cap? Will aluminum foil or plastic wrap work or is aluminum a bad idea? I would need something really thin as I don't have more than an inch or so under the lid of my mash tun. I don't recalculate my mash as I use an insulated tun instead.