Yep, that sounds about right...
1000 ICU = 1 gm/litre. 1 gal contains 3.785 litres. 5 gal is 18.925 litres. Therefore ~19 grams would be 1000 ICU, so to hit 500, you need roughly half that - 9.5 gm.
The thing that confuses me, is that Mimosa contains fruit juice, in addition to the zest. I...