Cuffey & Paterson does have good stuff on till so I should not be lazy and I should start using the latest edition ...
Use mask values to avoid averaging water thickness values from ice-free land or ocean (i.e. floating or ice-free ocean) cells, when computing the (staggered-grid) water thickness which enters into the Darcy flux calculation for subglacial hydrology.  Seems to have very small affect on nbreen.  Has small, and in every case error-reducing, affect on test P of -hydrology distributed.