There's no simple, fixed answer to what the reading from your meter represents in absolute radioactivity. The conversion from meter reading to picocuries will depend on several details of how your meter is built. For example, transistors have varying gains and are only guaranteed to be in a certain range. The sensitivity of your can and wire arrangement would have to be calibrated to find out its conversion efficiency from ionizing radiation amount to current.
So, what could you do to calibrate your meter?
Radioactive sources that contain very small amounts of radioactive material are sold legally to the general public. See http://unitednuclear.com/index.php?main_page=product_info&cPath=2_5&products_id=819&zenid=05b90526bbd4bb31df4d61ca8ac8be10
for example. You could buy a few of these sources and measure what reading you get from each one. There are several issues in what sources to get and how to present them to the meter, and other issues. Also, each source costs about $100. I don't recommend this method. If you can spend the money, you might get one source just so you can bring it close to the meter and see if its working.
A better way to calibrate the meter is to take Radon measurements in various places and compare these readings to known or estimated Radon levels.
Start by performing a Radon test outdoors. Run your collector outdoors and then take the reading. You can find information about what Radon levels are expected outdoors by doing a web search. Then run a test indoors on the ground floor; then run a test in the basement. Note how much the reading increases from outdoors to ground floor to basement and compare the values assuming that your meter reading is proportional to radioactivity. For example, if you find that the expected outdoor Radon level in your area is 0.4 Pci/L and the meter reading is R, and then the meter reading is 2R on the ground floor, you expect the Radon level there is 0.8 Pi/L.
To further calibrate your meter, you could take a reading in some location and at the same time expose a commercial test kit. Send off the kit for an assay and see how it compares to your reading.
Hope this helps. Get back to us with any other questions.
Good luck, WW