Back again, just to confirm im on the right track.
My imaginary defect is 1.5mm in height and 6mm in length, so its a 0.25 ratio?
a (depth of defect) = t x 0.055 = 1.375mm
l (length of defect)= a(1.375) x 4 = 5.5mm
Thus anything over these is a rejectable defect so my 1.5mm x 6mm is...