Underdosing might not be a good idea. Cyano is more like a bacteria than an algae. Under dosing may result in resistance developing, similar to what happens with antibiotics. Why don't you just shut down your refugium, treat it, do a water change in the refugium, then put it back in operation.
I don't know what this stuff is that you have, but it might actually be an antibiotic. If its old it might be out of date. You might want to just toss it and go get some Chemi-Clean.
If its just a little cyano, you might just want to do nothing and keep an eye on it.
Gary
Gary
125 SPS, 75 gal. LPS/softie reef, 9 gal. Nano