just remember 'wind on millibars, wind on altitude'
So with a QNH of 990 when you get to 3000 ft and twiddle the knob from 990 to 1013 your altimiter will wind up from 3000 to 3621 (23 mb * 27ft). Therefore the first available FL above 3621 ft is FL40.
I'm doing Met at the moment and it has a lot of questions like that, but in more detail. Draw a little diagram for each question and it all becomes clear.