(1) When all other conditions are as stated in section 67, with the exception that the slenderness ratio exceed 12, but does not exceed the values set out in Table XIX, then the stresses in masonry shall not exceed the basic stresses set out in section 67 multiplied by the factors set out in Table XXI.

TABLE XXI

(2) The factors corresponding to intermediate slenderness ratios shall be found by linear interpolation.