Nice picture !
What means 20 minutes of lights ? 10x2 minutes or 2x10 min or a single of 20 minutes ?
10x2 minutes != 2x10 minutes.
The exposure time per frame is the most important, but the noise in the final photo is the inverse of the square root of the number of frames, so 16 frames makes only a quarter of the noise.
So in my opinion / experience the 'real' combined exposure time is
(exposure per frame) * sqrt(number of frames).
In all cases, you can get much more from the original frames you already have. After stacking, you can add +1 or +2EV to the exposure in postprocessing and then process further. But this is a good beginning !
That is called the ETTR (Expose To The Right) approach which means that the histogram should not be too far left and from the other hand there should be no clipping (which presents as 'burned in' brighter stars).
So don't be too shy on somewhat more exposure.
Check this page:
http://www.clarkvision.com/articles/index.html#part_3