So my tires are
29x3 Chaoyang Big Daddy, and the tubes are 29x2.4-2.8 that I bought off eBay, presumably from a guy with a bike shop that is accumulating new tubes from selling new bikes and converting them to tubeless.
At 15psi, it doesn't feel from the outside like there's any slack, but I guess my fingers are useless calipers. When I'm rolling over rocks and roots and the tire is deforming, I guess anything could be happening inside there