We use an equal part of a 15%glycerol in Iscoves MEM and the harvest media, 2% FBS MEM, for a final 7.5% glycerol.  Our pools stay in good shape for 5-10 years, but we use them up way before then. 

The glycerol is autoclaved, then a solution of 56% is made with DPBS and sterile filtered.  Anything more and your filter will plug up. This can be made weeks in advance and stored @ 4C.

The diluted Glyc/PBS is mixed with ISCOVES MEM for a final 15% glycerol concentration.  This can also be made weeks in advance.

When harvesting, the viral supernatant is mixed 1:1 with the 15% iscoves MEM. Hope this helps

40% Glycerol, but pH=7.0