I zero all my hunting rifles in using a 50 yard zero.
I have an IR hunter Mark 2, 35 mm, playing around with the contrast and brightness settings, on a bright sunny day with the sun at my back shining brightly on the paper target, I can make out the outline of the bold red center target of a Redfield rifle sight in target page.
Or a small piece of black duct tape works well also. Anything darker than the white page of the target seems to work well at gathering enough heat to show up. Beyond 50 yards I cannot see the target however.
If I don't have the luxury of having bright sun to work with, you'll need to use some sort of artificial heat source for best or easiest results.
Hand warmer, hex nut that's been heated with a torch, etc. A bottle of cold water works well on a hot day for one shot.