Those strips are about as reliable as dipping your finger in to test the pH.
You should buy a decent pH metre.
I use cooled, boiled water for my Starsan, it turns cloudy very soon after mixing and the pH is always around 2.6, which is under the pH of 3, as recommended by Five Star to be effective.
The pH stays at that level for months.
Below is a copy and paste from
Five Star Chemicals product tech sheet for Starsan.
Homebrew use: Thoroughly wash all surfaces with detergent or a compatible cleaner, followed by a potable water rinse before application of sanitizing solution. Prepare a use solution of 1 oz. of Star San per 5 gallons of
tap water. Apply on surfaces with a cloth mop, sponge, spray or by a 5 minute immersion. For spray applications, use a course mist, with pump or trigger spray. Spray 6 to 8 inches from surface; rub with a brush, cloth or sponge. With spray, cover or remove all food products. For all applications, allow to air dry, however surfaces must remain wet for at least one minute.
In my opinion the often repeated mantra about using distilled water is safe to ignore.