So that means about 1/10 ounce of concentrate (3 ml) diluted to 20 ounces (640 ml).
In order to "calibrate" your dispensing system, take an eye-dropper or bottle with a drop dispenser and use it to count how many drops of the Photo-flo are needed to dispense 1 ounce (32 ml). If the answer is 30, you know that you need 3 drops to dispense 1/10 ounce (3 ml).
You can pour the 1 ounce back into the initial bottle.
Like many here, I find 1:200 to be more concentrated than necessary, so I would suggest that if your calibration suggests 3 drops per 200 ml, start with 2 drops first.
Be sure to mix it well - I like to put the concentrate into a small amount of water, and then dilute it to my target volume.
There are two further things I've found helpful.
1) A small (4 ounce?) bottle with an eye dropper incorporated in a re-closable cap is very handy - just dispense the Photo-flo concentrate into that first. Remember that you will have to calibrate it as well, as the dropper may dispense larger or smaller drops; or
2) Consider making up a "stock" solution of Photo-flo in alcohol.
For the last couple of years, I have been doing the latter. I have an 8 ounce (250 ml) bottle that I use to mix up a stock solution of 25 ml Photo-flo stock diluted with 70% isopropryl alcohol to 250 ml. For use, I further dilute that stock solution to a working solution by adding 23 parts water to one part stock (usually 25 ml diluted to 600 ml working). This means I end up with a total dilution of about 1:240, which works well with our water.
It is much easier to measure 25 ml reasonably accurately than it is to measure the concentrate in drops. The alcohol in the stock solution prevents the growth of mould.
Hope this helps.