How are the intensity values calculated using the saveblips plugin?

The intensities are area-weighted averages of the sample values (i.e. samples farther from the radar get counted for proportionally more, because the "sector" they represent is proportionally bigger). That average is then divided by the maximum possible sample value (4095) to get a number from 0 to 1. It's meant to be independent of the particular radar set-up (e.g. 12 bits vs. 8 bits of raw data)