A lot of confusion concerning DPI.  Consider, DPI is pixel density, and pixel resolution is image size.  If you are going to look at a rendering close-up, or the subject of the rendering has high detail, then you might(?) want a high DPI -- pixel density.  If you are going to stand 12' away from a 6' by 3' image, then density will/can be low.  The same resolution image in a 12" x 6" print size, seen from 2' away, will have a much higher print density, but will look much the same to the human eye.

DPI is more a human factors issue, pixel resolution is the amount of total image data, based on the native image (which would include color data).  Pixel resolution is based on objective criteria, DPI is based on human eyesight criteria (how big will the print be seen...at what distance).

There is not objective basis for determining DPI with a 16:9 ratio -- that is just a ratio and has no bearing to DPI or resolution. So, you start with 16 x 9 pixels as the smallest whole number image size.