I tried that before last time when I was working with JVC on video cameras.

A few considerations when making DIY grey cards.
When printing on matt paper, just note that the grey cards/professional charts you buy have a certain reflection rate.

Remember that certain paint can absorb light. Also ensure that the reflection rate is at a 90degree angle so that means when printing on the paper/cardboard the paint has to be sufficiently level.

Next item to consider when using photoshop, make sure that photoshop is calibrated to the printer you are going to the video studio to print at.

But still the paper being printed on, we can't just use any photo paper. No matter what kind of % setting which we can set in Photoshop, once printed using varying ink and photopaper, the whole effect would be off already.

But if I am not wrong, a Kodak grey card doesn't cost that much either. I think it was in the region of RM38.