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I really appreciate the members comments above where it is emphasized that you need to feel what is happening; add shellac, pumice and alcohol as needed.

I think the Millburn brothers, who wrote a truly wonderful guide for french polishing, led people wrong with their very strong statement of getting rid of the shellac bottle while executing a pumice fill. On my first guitar I worked many hours with almost no results when I tried to pumice filled following their pumice fill description; I then watched Louis Fernandez DVD French Polishing for Guitarmakers and pore filled the entire guitar in less than an hour. Louis Fernandez method violated every admonishment put forth in the Millburn description such as more shellac and pounce bags for direct application of pumice when needed. The finish on that guitar still looks good 8 years later with no pumice showing in the pores.

I am sure the statements about not using shellac once you start with pumice was a counter to a tendency of people using too much.
I am sure the Milburn method worked for them, although I heard they may have switched to epoxy, But a strong statement like they made had to depend on the right amount of shellac left on the guitar when applying the spit-coat. If there is not enough shellac to bind the fill inexperienced finishers are left wondering if somehow their technique with the pad is wrong. In that case the only way to go forward is to add more shellac.