Well, tomahawks are generally vastly overpriced ime. Easiest thing to do is just weight the bone after you cook the steak. It’s not really a set ratio. Some ribeyes are bigger than other. Fatter cow, more marbling, bigger cross section of meat, but the bone probably isn’t any bigger. I really don’t think here’s a hard and fast number for this.
Generally speaking, it’s about 4oz of bone on a KC strip, but I couldn’t tell you how much a tomahawk rib bone weighs usually as I can’t remember the last time I worked with them.