It's a normal aerodynamicists shorthand to refer to temperature, pressure or density at altitude as a proportion of the conditions at sea level - as you go up pressure drops, temperature drops, and density drops. So, whilst at sea level everything has a value of 1 (or 100%) it all drops as you climb. In a pure vacuum (which doesn't actually exist anywhere) density and pressure would be 0.
When an airliner is cruising at altitude, both the pressure inside and the pressure outside the aircraft are less than sea-level conditions. In pressure terms, inside it can be as high as 81% of sea level pressure, and outside it can be as low as 12% of sea-level pressure. So, the difference between the two (it's always the difference that matters, not any absolute pressure) is 81%-12% = 69% of sea-level pressure.
At that point, I multiplied my 70% (69% rounded off) by actual sea level ABSOLUTE pressure, which gave be the difference in pressure between inside and outside the aeroplane.
The actual values I took from a data book that lives on the bookshelf behind my desk, but it's never let me down yet ! It was "Aerodynamics for Engineering Students" by Houghton and Caruthers, which is about 15 years old and a little crude, but good enough for this sort of crude guesstimate.
Hope this helps.
G