Assuming the following typical conditions:
- natural gas manifold pressure is 3.5" w.c.
- propane manifold pressure is 10" w.c.
- natural gas and propane specific gravities are 0.6 and 1.52 relative to air
- natural gas and propane energies per cubic foot are 1038 and 2516 BTU
- your orifice coefficients are the same
then the propane orifice diameter should be 0.623 times the natural gas orifice diameter.
