to obtain the desired line height type n, where n is the number that will be multiplied by the element's font sz(just a number with no units)
font-size: .9375em; / 15px/16px/
line-height: n; / 1.65 x 15px=24.75px /
To set a line-height that is n times the font-size, you can do:
font-size: .9375em; /* 15px */
line-height: n * 0.9375em;
So for example, for a line-height 1.65 times the font-size:
line-height: 1.65 * 0.9375em; /* 1.65 * 15px = 24.75px */
Or using unitless values:
font-size: 15px;
line-height: n * 15px;
So line-height: 1.65 * 15px; would give 24.75px line-height.
In short, just multiply the font-size unit/value by the n factor you want for the line-height.