I used a test plant throughout...a runt female in the back of the grow room that I always hit really hard with things first===like three days ahead of what I fed the others. I wouldn't risk leaving a known male with your girls, though. Go super easy with the cactus juice. Since I abused the runt with testing nutes, she has tall, skinny buds that are airy and have almost no weight.

HD always said that good soil eliminated the need to use a whole lot of ferts.