I always use 2" more rubber per 10" of diameter when mounting. So a 50" wheel will have 10" more rubber than the circumference, wrap the rubber around the wheel and overlap and cut 10" past the other end.
Sorry to have to report a difference in experience. We usually have a 4 inch overlap for the big wheel, plus a minus just a little bit for sizes more or less than a 52 inch wheel, which was the most popular size in the 1880s. If you have 10 inches of overlap, I would think is would be impossible to compress that much rubber; however, if you are using some of the plastic tubing, it might work. Gook luck and let me know how it works, especially the kind of hard rubber you are using. Check the Wheelmen Bulletin No. 4.
I personally have used the formula of 1" overlap per every 10" of wheel diameter and have not had the tire butted ends separation problems due to heat build up from friction in the rubber during riding. Separation of butted ends is also caused by the tiring "snagging" on something in the rim cavity like a spoke nipple that has the spoke threads exposed beyond the nipple head or a headed spoke that is sharp or not seated down in the counter sunk spoke hole in the rim. Always grind down exposed threaded spokes extending beyond the nipple head and sharp headed spokes. I usually use a cloth rag and run it around the inside of the rim to check if it snags on anything. Sometimes the inner surfaces of the metal rim will have sharp places as well that need attention. Rim and spoke preparation is critical before installing a tire because in reality the tire actually migrates around the rim over time from riding the bike no matter how tight tire is on the rim due to the calendaring process of the rubber being compressed while supporting the rider's and bicycle's weight at the pressure point on the road surface. The tensioned wire inside the tire also migrates from riding the bike as I have seen this when removing old tiring and the wire joint can sometimes be half the way around the rim from the butted tiring joint. Smoothing rough edges at the solder joint of the wire (or twisted joint wire ends if done that way) will stop the wire from snagging in the soft rubber which can also be a problem.
After cutting your tiring to a rough slightly oversized length, my favorite tip is to grab each end of the rubber tiring and tightly hold them together so the ends are matched up like a double barreled shotgun and dress sand the ends simultaneously on a disc sander that has a work table. The logic for this is by holding the ends this way while sanding, even if you are not quite square to the sanding disc, it is not a problem as the ends that were sanded simultaneously now become the complementary angles to each other when the tiring is placed on the rim and will butt up dead flat and is visually neater once compressed on the rim via the wire tension. A 60 grit sanding disc works great and don't use too fine of a grit as it will load up and burn the rubber instead of cutting the rubber.
Hope this helps you solid rubber tire bike riders.
Mike Cates, CA.
Our rubber is made using an analysis of some of Dick Hammel's tiring by our factory, so you can be reassured that our product is made, as closely as possible, as the rubber which we have been using over the many years, Dick supplied us with rubber. From my experience since last year, the new rubber is as durable as the rubber made by Dick Hammel many years ago. Some of my bikes still have the original rubber we purchased from Dick back in the 1970s. For these reasons, I would check out other sources which offer buggy rubber or lab tubing and ask for their data on their product.