Im not sure if any of us can tell you wether or not your going to fall under the 20ng level. Im really not sure why your cutoff would be this low for a regular pre-employment drug screen. The only tests ive seen that fall under this category would be a DOT test or and military screen.
The THC cutoff level for all DOT tests is 50 ng for the initial screen, and 15 ng for the confirmation test if the sample tests above 50 ng on the initial screen, as outlined in Subpart F - §40.87 of the DOT regulations, which can be found here: http://www.dot.gov/ost/dapc/NEW_DOCS...rt_f/40_87.pdf