I think it's .0005" CHE, that's "ten thou" not thou.
Of course this is all relative and can vary a significant amount even if using the very best techniques. For decades this was considered a very good indicator of pressure but even the greats dissuade its use unless the inherant large margin of error is factored in.
.0003" CHE may tell you that your load is reaching/reached or beyond a max load, and that's the problem, you don't know where but you know it's in the area. I have measured loads that were near .001" CHE and then and only then were they approaching 65,000 psi on actual test equipment, then there were others that never even approached .0005" and were already running 65 kpsi. The baseline measurement was taken on once fired brass, not virgin, but once fired with medium load. The baseline case average diameter was then compared against the diameters from the same cases fired a second time. Beltless cases were measured immediately above of the extractor groove, or on the belt for a belted case. All case were measured relative to a specific mark on the casehead so measurement were always taken in the same place.
Pressure ring expansion is even a more accurate/consistant method but requires a blade mic and a method to control distance from the case base and where the measurement is taken in order to be accurate. A proper baseline etc.
Southwest Products PressureTrace is a very inexpensive method to attain the real pressure numbers, no guessing game. If you can test fire next to a computer (laptop most portable unless you shoot out the back door), this will work for you.
Here's a load I worked up using PT. Can you spot where the blast from my rifle shook the 5.0 gr slider weight on the beam scale I was using over one notch as it sat in the warm up shack three benches away where I was loading one round at a time? When I spotted the pressure increase and also the MV jump I went to investigate the scale. From that shot there, it had just jumped another 5.0 gr! Thank God I caught it when I did.