This is the protocol concerning the PEI step for the NCp7 purification:

Make up a 4% soln. of PEI (Stock is 50% PEI, Sigma P-3143):
Take 1ml of the 50% soln. and bring up to 12.5 mls with 50mM Tris pH8.0,
0.1M NaCl, 0.1mM ZnCl2, 10mM bMe, 10% glycerol and pH to 7.5 with 12N 
HCL).

For a 40-50 ml lysate (from a 2 liter culture) which has already been
resuspended and sonicated, add 4mls of 4% PEI dropwise (slowly) while
stirring on ice. Have the addition of the PEI take about 15 minutes or so.
If you have a 4 liter prep with a 100ml lysate, then use 8mls of the 4%
PEI. I think the key is to have the lysate resuspended well and not too
viscous. That's why the lysate is in a fairly large volume. If you took
the OD260/280 after 1ml of addition, 2,3,4mls of addition, you would see
that the 260/280 ratio will stabilize at approx. 1.5-1.7, but the total OD
will decrease. That's normal. After addition of all the PEI, spin out the
insoluble material and put the supt. on the column. It seems that most, if
not all, of the NCp7 stays in the supt., so you actually get some
purification right there.