This rule and SES dialogue are intended to point you toward either:
1. bolting through welded inserts centered in the front bulkhead tubes, or
2. using gussets to react bending loads on AIP mounts offset from tube centerline.
In case 2:
See the diagram on the EV Accumulator tab in cell AY28.
In the front view, measure the distance from the tube centerline to the tube surface.
It's not a 3D measurement.
Preventing bending forces along the shear plane:
Length is the weld length parallel to the tube surface you used for the offset.
Thickness is material thickness.
Preventing bending forces along the fastener axis:
With no gussets behind, the values above are reversed.
Otherwise, add up the gusset thickness and use the minimum gusset length.