24Champ, the best way is trial and error!
I found out the hard way, too much ginger in the morning made me wanna throw up. I love ginger, but I put too much in on 2 different batches, and both times really felt like I was going to hurl.
I take blood pressure medicine, so no grapefruit for me, but I loved it when I did eat it before!
Here's an old link or two I found some tips on too.