Typically, I will dump the water from the rinse and then add 2-3 drops of photoflo to a 20oz tank and fill it, adding a little extra water after it's full/shaken to wash some of the extra suds away.

Ratio-wise, it's probably less than 1:1000, but I've never had any issues. I think 1:1000 is a good, safe bet as long as you rinse well enough beforehand. The stuff is used so sparingly I've never gone through a bottle before its printed expiration date. And while I can't imagine it ever going bad, it's so cheap you don't need to worry about using a little excess every time.