You can always use the calc function for this.
Syntax:
calc(expression)
Eg:
abc {
width:calc(100%-20px)
}
Here are the list of browser that supports this function
EDIT 1:
you can use it in the following two ways:
LESS Input:
@bpMobile: 600px;
max-width: calc(~'@{bpMobile} - 1px');
CSS Output:
max-width: calc(600px - 1px);
2nd Way:
Less Input:
@bpMobile: 600px;
max-width: calc(@bpMobile - 1px);
CSS Output :
max-width: calc(599px);
With the first option,the calc arguments are escaped in order to prevent them from being evaluated on compilation.The values will remain totally static until it's evaluated by the client.
With the second option, the calc value will be evaluated on compilation. And it would be the same as
max-width: @bpMobile - 1px;
which will result in
max-width: 599px;