Any method you choose to use could be considered "legitimate". Whether it's common or accepted practice is another question. By not clamping the strip to a holder the strip is allowed to relieve itself during the peening operation and therefore the arc height achieved will be different. This is the same phenomena experienced with roto-flap peening when the Almen strip is attached to a special magnetic holder which allows the strip to deflect during the peening treatment. I have heard of another practice where one side of the Almen strips had a tape applied to it instead of being mounted on the standard Almen holder. Again, the readings are going to be different than if they were constrained on the holder. This practice, although novel, can also be considered useful in that it is intended to demonstrate process consistency from day to day. Your method could also be useful and therefore valid. However, if you have a customer that is specifying the intensity according to SAE or other standard practices you would have to ask for a waiver to use your "hybrid" method.
Jack
