not sure about 2016,
If you RMC your mouse on the semi circle you can select 90 or 180
check your spin increments in system settings, could be set at 15deg.
Thanks for the response Frank.
I didn't notice this in 2014 or earlier versions.
I did try the RMC option for 90 or 180 and that work as expected.
My spin increments are set to default 1 deg.
The inaccuracy is consistent when using the large hash marks or the small ones to to land on what should be a whole number. It's always .5 deg off.
It's not so much that can't get the rotation I need another way, just making sure I'm not missing something with this feature.