Greg Esres wrote:
The difference between calibrated and indicated are within only 1-2
knots.

Around the stall speed, the error is normally much higher. When you
take sqrt(3.8) * Vs1 to get the hypothetical Va, you will normally get
a very different number if you use IAS vs. CAS.

I am using Vs (stall, clean) since the configuration is the same as at Va.

I thought it was true airspeed that mattered?

No. Aerodynamic forces depend on CAS, rather than TAS.
As others have pointed out, it is permissible for a manufacturer to
set Va at a higher speed than what you will calculate using this
method. Most do not do so, however, but it's unclear what is guiding
Piper. Some of their POH's show stall speed as linear with weight,
which is blatantly untrue. If you start with this premise, Va is
likely to be off too.

How does CAS account for density altitude?