Search on Shazzam's posts. He did great bit on exactly this a while back. 19x18 for twin four-pots, and 16x18 for a single as a basic rule. Depends on whether you want more initial bit or progressive feel.
So the formula is the area of the calliper pistons divided by the area of the master cylinder.
A std fireblade is around a figure of 34.5. An nc 30 is around 31.5.
Brake levers are about feel.
Too great a number and you have a hard lever, but no feel.
Too low a number and you have loads of feel but of course too much travel!
I would suggest from what your saying Frank, your master cylinder is too small a bore to cope with your 6 pots. ( what a std 16mm ducati master cylinder bore?)
Thats easy to see when you think of the amount of fluid required to move all those extra calliper pistons.
I bet your ratio is low 20's.
Brembo 19/18 master cylinder to sort me thinks.......