I think Ransley's numbers are good. Fiberglass nominally has an R
rating of 3.5 per inch. 6" is, therefore 22. Until you get to 60 you
can just keep adding more, PROVIDING that you don't block ventilation

Umm.. sort of.. The bottom line is, you can't save more in
heating bills than you're spending. So you figure out how
much you're paying in heating bills, and which surfaces account
for how much of that. If you can double the insulation in
any given area, you halve the heat loss there. That gives you
dollars per year saved. Multiply that by how long you expect to
live in the house, or 20 years, whichever is smaller.

If you can double the insulation for that price or less,
then it makes sense to do so. If you can't, it doesn't.

northern NH, the cheapest insulation is blown in cellulose which is
fine for horizontal surfaces like an attic. Usually you buy the
cellulose in bales and get loaned the machine to blow it in.

If there is ANY connection between the attic spaces and the
living spaces; a door, a hatch, can-lights, balloon framing,
then think carefull about blown-in cellulose. It will *NEVER*
stop sifting dust into the rest of the house.