Configure CQP (CRF) for h264_nvenc
from reading:
ffmpeg -h encoder=h264_nvenc

and the nvidia nvenc examples at (https://devblogs.nvidia.com/nvidia-ffmpeg-transcoding-guide/), I've arrived at:
ffmpeg -vsync 0 -hwaccel cuvid -c:v h264_cuvid -i test_input.mp4 -c:a copy -c:v h264_nvenc -b:v 0 -cq 1 test3.mp4

where:
-b:v 0 over-rides the default 2mbps bitrate as noted by Gyan
-cq 1 means target quality level 1 (highest)
 range of 0-51. 
 0 means automatic (in my case around 15mbps)
 1 gives about 15mbps, 26 results in around 5mbps, 51 results in 0.5mbps