Just the length of the exposed edge. 13370 refers to P as the 'exposed perimeter'. Note that according to ISO 10211, B' = 8m works for any floor where B' ≥ 8m, so in most cases like your example, you're ok to model 4m on the interior (1/2 B') and 20m on the exterior (2.5 B'). Hope this is what you're looking for (and it's not too late!).

John Mitchell
Partner, BLDGTYP LLC