If I draw an imaginary character and use a photograph as reference for human body/pose, is that legal, copyright-wise? Let's say I change the hair and the face? I know it needs to be 80% different, but not sure how much % a figure pose is worth.
Since you are drawing your own character and the photo is for reference only it should not be an issue. To be sure use multiple references. That will make it so no one could point to your drawing and identify their own work within it. It also gives you more data to draw from.
A process I like to use is gather a bunch of reference photos, then do a series of sketches based on them before executing the final drawing. Then you would be working primarily from your own drawings, with the reference photos there just to resolve and questions of proportion or form.