BIAB Efficiency of mash - how to work out?

[daf]

Apologies if this has been answered many times before - but I read a lot about people with percentage efficiencies - how is this calculated?

I mashed 35 litres at 66 deg C yesterday - 4.5kg crushed Pale Maris Otter plus 95g of Light Crystal Caramel malt.

Gravity of the wort pre-boil (32l) was 1.044 or thereabouts.

[daf]

Sorry - lazy me - I found this: http://www.brewersfriend.com/brewhouse-efficiency/

[daf]

Efficiency:
96.83%
Points / Pound / Gallon (ppg):
36.7

[AnthonyUK]

I did something reasonably close to that too this morning and ended up with 1.044 pre-boil

[daf]

I guess I'll keep mashing in the kettle rather than in a separate picnic cooler!