Citation for this was hard; the closest I got was Etzioni's 1962 The Hard Way to Peace, pg 110. There's also a version in the 1998 Linus Pauling on peace: a scientist speaks out on humanism and world survival : writings and talks by Linus Pauling; this version goes

I have made a modern formulation of the Golden Rule: "Do unto others 20 percent better than you would be done by - the 20 percent is to correct for subjective error."