I use one or another old kodak Dataguide. I have one from the early 70's like the photo above that Ron has posted, and one from a leter color darkroom dataguide that has 8 white to black squares over 8 different color squares. Both are opposite a grey card page.

I like the second variant, as when shot so the image fills the frame the resultant squares are large enoygh in 35mm to be read by my densitometer, which I use in lieu of running control strips to determine my batch to batch chemistry aim of uniformity. I have a few rolls of agfa film shot of just these targets in the fridge, and I snip off about 4" worth and load it in the tank along with the first reels thst I process from a new mix of developer, etc.