Dunno how much of 'ease of working' comes from 'time spent using'.
I guess it's easy to suggest that once you are used to working in decimal systems (mm, cm, m, km, etc.) then it's easy to guess (thus helping with the maths - in the cases where conversion is required and one can't/it become infeasible to just continue to measure with the same unit) for other decimal systems (ml, cl, l).
For me, outside of my work (where super precise measurements are required), I rarely ever have to switch between yards and feet - or yards and miles - or stones and ounces...
So I can get away with saying 1.5 miles, or 300 feet away - or 12.7 stone - or 6 foot 11 (though, this is kinda an exception as it uses both feet and inches - it's a simple base 12 conversion [which people do without thinking - as the duodecimal system is king])