I have to correct you, folding wings, the Vulcan did not deliver retard bombs but ballistic 1,000 pounders. Retarded bombs, in any case, would not achieve the required penetration to create the runway damage required.
As for the calculation used to achieve best results, stick spacing and anticipated crater diameter determined the cut angle to deny minimum clear width and weapon release was calculated to hit the runway with the middle of the stick.
In fact, as is well documented, the release error of the first Vulcan attack was such that the first bomb of the stick hit the far third of the runway and barely denied minimum clear width. However, the strategic effect was not only to persuade against use of the airfield by fast jets throughout the war but also to redeploy mainland based aircraft in defence of BA. A good night's work.
Much has been posted about the Bucc's oil consumption but the F4 with two speys flew non-stop to Singapore in the early 70's so that was not the reason it was not used. Both Bucc and Tornado were considered but the Vulcan was selected. Incidentally, the Nimrod crews also looked at bomb delivery and in training achieved better results than the Vulcan crews.