Yes Sir. You already know, but I want to say it anyway for the sake of discussion...stir/shake Hardener very well. It's PVC-concentrated so it will settle with the best of 'em.
I believe different brands of Hardener will likely vary, but off the top of my head, I'd add 1.5 ounces (3 TBSP) Hardener to 12 ounces of Regular to get something like the average Hard. That ratio would be a squirt over 10%, which I'm applying to the assumption that You're using a Hardener that is easily shaken and poured like standard plastisol. There ARE very dense Hardeners. If You're using one of these thick, dense Hardeners, I'd start with 1 TBSP (0.5 oz.) to 12 ounces of Regular and go from there.