The listed suggested ppm levels on your bottle of Dyna Grow is assuming that your water is at 0 ppm to begin with. if you start off at 260 ppm with your tap water and the Dyna Grow says to use 400 ppm, then you must add 400 ppm of your nutrients so your level would be 660.

I would suggest you try to use distilled water, or reverse osmosis water which will have 0 ppm. Your 260 ppm tap water is probably ok, but may contain chlorine or calcium, or sodium or something else harmful to your plants. If you must use tap water, they sell hydroponic nutrients that are meant to be used with tap water and ppm levels around that 260. Flora series by General Hydroponics sells a "hard water" version of their 3-part nutrient line which is meant for tap water.

give it a shot with your dyna grow, just keep close watch on your pH level, keep it around 5.8, change your reservoir every 7-10 days topping with fresh water every day. Good luck!!!!!!!!