It is agreed that Roark's formula (7th edition) Table 9.2 Ref. No. 1 is not applicable for base block check in case of single tailing lug. Ref. No. 20 is the one that is commonly used by design software packages.

But, in case of 2 tailing lugs, what should be the case? Ref. No. 4 is used by PVElite, however, this looks like wrong. It is similar to Ref. No.1 in case of single lug.

Since, I'm preparing some excel sheet for the several cases of the base block configurations, I'm confused with the above.

If Ref. No. 4 is not correct, can we make a supper positioning by considering (Ref. 4 - Ref. 20) will really represent the case?