Neither grapefruits nor avocados are products of exact weight, and there is no need to maintain strict proportions in such a simple recipe anyway. A salad will be tasty and perfectly balanced if it contains approximately equal amounts of avocado and grapefruit. There may be more avocados, but not the other way around. If you multiply the grapefruits, they will completely overshadow the avocados. So, something between 2:1 two parts avocado - one part grapefruit and 1:1 equal parts will be delicious.