Hello again,  I am sorry for the many questions today...But..here's another...As I said I had gotten away from B.P. shooting...But It was still never far from mind..I managed to obtain most of Paul Mathews excellent little books.  He told of his use of grease cookies..mainly for hunting I think.  He first tried wax paper as a grease-barrier, between the soft grease-wad & powder.  Later, in an attempt to eliminate even the small amount of ash/fouling from the paper...He used .030" thick dental wax.  Wax sheet pushed over case-mouth, wad pushed down on pre-compressed charge, & very soft, grease-cookie pushed down on top of wax wad.  Veg. fibre wad placed in case & bullet seated.
    Has anyone tried these over-powder wax wads?  It seems just before I gave up on black..I purchased some of this wax..Perhaps this summer I will try this out.  Best of luck!

I have yet to find any advantage with a grease cookie.  My barrel is 36" long, and it is hard to get grease to the end, so I have experimented some with cookies to no avail.  I have simply not seen any additional grease in the barrel.  For me, it has made only loading headaches and reduced powder capacity.  The only way I can get moderate grease to the muzzle is to use bullets with many small grooves, like the Saeco designs.  (Bigger, deeper grooves shed their grease faster.)  This has pretty well relegated my rifle to smokeless.

I don't know if this helped you one bit, since your question seems to have been more about the barrier than the actual cookie. 

Personally, I don't like putting things in the cartridge that cannot be weighed or measured.  It is asking for inconsistency.