Have I got this radiation calculation correct?

So, I finally got something radioactive, an old bullfinch mantle, complete with thorium and asbestos string.

Now, assuming my Geiger counter is reasonably accurate (last calibrated in 84 I believe, yes it is a bit out of date), this puts of at most around 4 milliroentgen per hour peak (when the meter is right against the thickest quantity of the mantle).

So, that converts to 34.78 microsieverts, and according to the xkcd chart, a flight from NY to LA is 40 microsieverts, so in effect, holding the mantle in my hand for 1 hour would expose said hand to just under the amount of radiation my whole body would experience in one flight.

Also, based on that, I think that I would need to hold the mantle in my hand for 1,437 hours to reach the safe annual radiation dosage for radiation workers, or 966 hours for civilians (at least in that hand)

Just wondering if someone can confirm I have got that all straight, or perhaps I made a mistake somewhere.

http://xkcd.com/radiation/
http://www.convert-me.com/en/convert/radiation/