Are you a student? Just wondering.
"where the SPT stops" ? Does this make sense if you have a 2 m wide footing and the SPTs go down 20 m?? No, it doesn't.
There are two issues - one is bearing capacity which is a shear phenomenon; the second is serviceability - a fancy term for settlement (or limiting settlement - including differential).
I would say - what is the depth of influence of the shear planes below the footing. See Das' Principals of Fdn Engineering, 7th Ed (pages 137-138) it is explained how the shear planes are formed. This is the zone in which you would need to determine the characteristic/acting phi for your bearing capacity.
However, given the above, it is seldom a controlling factor in the size of the foundation as the foundation size is almost always controlled by settlement issues - either a standard "acceptable" settlement such as 25 mm (traditionally 1 inch) or 40 mm (as per Indian practice). However, there are other times that more stringent settlement constraints are required. In these cases, it is usual to take the "zone of settlement influence" to be that location where the Bousinesque pressure increase is in the order of 10% of the applied stress. Typically for square or round footings this is 2B where B is the width of the footing. For strip footings, it is typically taken as 4B.
