You need a device that can measure the different types of radiation. Then you have to do a bunch of calculations to estimate absorption. Only then can you calculate Greys which is the measurement that matters.
PS 300 CPM is nothing. There are places where people live where the natural background radiation is higher than that. Also, background radiation is mostly Gamma rays which is more dangerous than what comes off of fission products or nuclear fuel.
That depends on the instrument. It's an instrument-specific rate. I'd say that saying "300 CPM is nothing" absent any information other than the CPM is foolish.
As I indicated earlier, given all of the other context we have, we can reasonably suppose that 300 CPM from whichever instrument was used in this incident is nothing to be concerned about.
PS 300 CPM is nothing. There are places where people live where the natural background radiation is higher than that. Also, background radiation is mostly Gamma rays which is more dangerous than what comes off of fission products or nuclear fuel.