Digital BW, The Print

I'm in the process of switching over my Epson 4000 from UC inks to MIS 
UC Equivalent inks (including the new MIS PKN-neutral Photo K and MIS 
UT7 LK-also neutral).  Does anyone know what volume of ink is in the 
lines that go form the cartridge to the head?  I want figure out how 
many purge pages will be required to flush the PK and LK lines when 
printing either 100% K or LK.  I can get the ink volume used per page 
from the 4000 printer utility, but need to know the volume in the line. 
  If there is a very large volume to purge then I could just do a power 
clean cycle, but that will waste a lot of the color inks as well.

Carl