You should try to calibrate your backpack spray setup so that you know how much you are putting down over what area. That will help you determine how much to mix for your backpack. I personally have 4gal cover about 5000 sq.ft. at a slow leisurely pace. I do use a 3 nozzle boom using blue Tee jet. I would use red but my sprayer is not capable of the output needed with 3 nozzles. Again, you will have to figure out how much a full tank is good for with a given tip and at a preset walking speed. I would recommend either the blue or the higher output red teejet fan tip. 80degree unless you are holding the tip low to the ground. I use 8oz of Lesco 12-0-0 iron plus per thousand sq.ft. Mix primo according to recs on the label--for my kbg I use 1/2 oz per 1000 sq.ft. I love the stuff, especially now that it is only $275/gallon.

I only use Primo on stoloniferous turfgrasses mowed less than 0.750".