Depends on what you are packing, but I pack bud broken into 1/2" chunks at the rate of 3.8 grams per cubic inch and 10 mesh at 4.1 gm/cu in column volume.
Unless you are subzeroing the column, soaking in a closed column extracts more non targeted elements like plant waxes, b-Carotene, and Pheophytin.
You are still open blasting, just increasing the soak time.
As far as pressure, zero pressure would be good, but depending on the LPG mixture, your pressure at ambient will be the same as the gas charts for that temperature and mix.