Then you need to ask what is the lowest Mach we are willing to admit and accept. :-) Not the minimum (limiting) value.
Careful though, the lowest Mach does not really exist. It is the lowest IAS while still in MACH (mode) segment that is limitng. The importnace here is it will be different for the same aircraft in different levels, .72 for my aircraft at FL370, .63 at FL 280 perhaps.
Not to mention different payload weight... (fuel is presumably constant mostly for arrivals to a single destination)