I assume you're talking about aqueous solutions. To make a 85% formic solution, mix 85mL formic per 100mL of the total (i.e., for every 100mL you have, 85mL should be formic, and 15mL water). To get 35% solution from a 50% solution, mix 70mL peroxide per 100mL (i.e., a 100 mL solution would be made up of 70mL peroxide and 30mL water). I wouldn't use tap water... I would use distilled/de-ionized water. Good luck peace