Over shot card

[Big D]

All other things being equal, I would think the pressures would be minimally higher due to the increased mass of the card

[edgarspencer]

According to Precision, my roll crimped paper shells, with overshot card, were consistently lower than my same charged, folded crimp shells.
An overshot card, used as a filler, atop the shot, then fold crimped, may be infinitesimally higher than without the card. ( A bag of 500 over-shot cards might weigh a few ounces.)

[Buddy Harrison]

If you're really concerned, do as I do. Subtract an amount of shot equal to the weight of the card wad and you're good to go.