First of all, Mach is already a percentage, the ratio of the speed of sound to your current speed at a given altitude. Limiting speeds are not based on a percentage of a percentage.
Second, V speeds do not change with altitude until you hit the limiting Mach speed, at whch time the Mach limit takes over. If you are looking at a chart where the Vmo is decreasing with altitude, then at some point on that graph you passed Mmo and you are actually reading Vmo base on the limiting Mach. .48 M is apparently that speed for the Beech you mentioned.
Dr. Mach discovered that air behaves the same way for a given percentage of the speed of sound, regardless of altitude. That discovery is why he got the speed named after him. Those same laws apply to all aircraft, not just high performance ones. A C-172 would have a limiting Mach if it had an engne powerful enough to go that fast.