Less no longer evaluates expression inside calc by default since v3.00.
Original answer (Less v1.x...2.x):
Do this:
body { width: calc(~"100% - 250px - 1.5em"); }
In Less 1.4.0 we will have a strictMaths option which requires all Less calculations to be within brackets, so the calc will work “out-of-the-box”. This is an option since it is a major breaking change. Early betas of 1.4.0 had this option on by default. The release version has it off by default.