Calibration / PPM

I am trying to get my head around calibration of my RTL dongle. I used gqrx with a known frequency (467.6375Mhz). Based on this, the dongle needs offset of -16Khz. In gqrx, this offset can be entered directly in the user interface. With rtl_test -p, the PPM number varies. I have gotten positive and negative numbers. When using the RTL block in gnuradio companion, I need to enter PPM. I have seen various formulas to calculate PPM from frequencies but for me they do not yield sane results. I could be just misusing these formulas. Any assistance would be appreciated. Thanks, John