How to set width/height as Percentage minus pixels in CSS

you can use calc:

height: calc(100% - 18px);

It’s worth it to note that not all browsers currently support the standard CSS3 calc() function, so implementing the browser specific versions of the function may be required like the following:

/* Firefox */
height: -moz-calc(100% - 18px);
/* WebKit */
height: -webkit-calc(100% - 18px);
/* Opera */
height: -o-calc(100% - 18px);
/* Standard */
height: calc(100% - 18px);