There are two things working here. How far the cones turn in get the proper cone-ball-race adjustment (very slight radial movement of the axle) and what is the correct overall dimension from locknut to locknut (a result of how far the cones are adjusted).
I guess I have to ask... what happens if you only use 8 new balls? Does everything work okay? There is an aspect that if it works it works and as long as there is not excessive radial or axial play and you can't use 9 new balls. I believe, and someone can correct me, that it is possible to use 1 less ball because the balls will just spread out a little more.
If you can't use 8 or 9 new 1/4" balls, I'm not sure what you can do. But if it were me I'd pick up some Campy 7/32" balls and see how 9 of those would work.
John