4th Week Into Flowering About to Add Ginormous But How Much Should I Add?

I'm into the 4th Week of Flowering About to Add Ginormous (just picked up a pint of it today) But How Much Should I Add? It says 5 - 15 ml the person at the grow store told me to add it every 3rd watering is this the best rule of thumb and also if I can't measure with a device is it just one or two tablespoons into a gallon?

A teaspoon is 5ml so a tablespoon or two would be too much. I would start with 10ml every third watering.

Oh sorry I meant to spell teaspoon not tablespoon. I started with a teaspon and a half so hopefully around 7 - 8 ml. I currently have 8 plants and I'm doing an experiment only adding this to 4 of them and seeing how much of a difference (if any) it'll make.