jJean,
4) At each arc height reading what would you estimate the coverage percentage to be on the test strip?
coverage is 100% since the first passage
You are peening the test strips way too long! At the point of saturation the test strip should be close to 100% coverage with peening coverage uniformly distributed on the test strip.
Your machine may not be capable of producing a 4-point saturation curve. It sounds like you may not be able to send the test strip though the shot stream fast enough to have anything less than 100% coverage on the strip. If true, then you have what is known as a Type 2 Curve. See SAE J443 "In some cases, saturation curves can appear as exemplified in Figure 2 (Type 2) and occur only when process variables do not permit the attainment of earlier data points."
