The recommended quantity from the manufacturer is 1 gram per gallon. IIRC somebody weighed this stuff and it's around 85 grams per cup, so basically in your case, roughly 1 cup by volume (or half of the can if you're buying it in the 150 gram size).
A lot of folks just load up the reactor so they don't have to refill it as often. The "standard" size reactors like the TLF 150 or the Kent reactor are intended to handle a full 150 gram size container of GFO (roughly 2 cups). IIRC, this fills the reactor about half way.
Set the flow rate so that you see some movement of the granules near the media surface, but not so much that they're tumbling.
Personally, I rinse this stuff before I use it. I also catch/dump the first few cups of water that comes out of the reactor after I have it hooked back up.
One word of caution... if this is an established tank with sensitive critters like stony corals, etc., you might try starting of with a small amount - say a couple of tablespoons. I've personally never had a problem, but some folks have reported issues when bringing elevated phosphate levels down quickly in an established tank - particularly with stony corals.
HTH