I say you need to take the zone I negs to a place that has a transmission densitometer. The correct density is .10 above or minus film base + fog. That means subtract the clear frame density from the ones that are suppose to be zone I. Once that is found, use that for your ei and re-shoot the gray card but open up 3 stops. That should be zone VIII. Once that is done measure the z VIII frames on the same densitometer. Zone VIII is 1.20 minus fb+f (This is assuming you are using a condenser enlarger) (on the chance that you are using a diffusion type enlarger the density should be no greater than 1.30 minus fb+f). This may take you several attempts to get there and you need to make sure the temp of the developer is what you think it is.
The reason you need to use the densitometer is there are way too many variables to deal with when you make the Max black tests. With the densitometer it is pretty cut and dry. Look for a custom lab in town and call them and see if they will read your negs. Another place is a good size print shop. Just make sure they understand that you need to read the negs on a transmission densitometer.