How is it possible to quantify the strength of a Lewis acid/base?

When the $pK\_a$ scale is not applicable - when there is no proton/charged species transfer - what should be used to quantify the strength of an acid?




$$\ce{A~ + :\!\!B -> \ \ A\!\leftarrow:\!\!B}$$


$$K\_f = \dfrac{\ce{[A\!\leftarrow:\!\!B]}}{\ce{[A][B]}}$$


See sections 4.1 and 4.2 in the following reference for more information:


<http://goicoechea.chem.ox.ac.uk/teaching/abse/abse_lecture2.pdf>


But $K\_f$ is specific to an acid-base pair. 


A more universal scale is formed by the [Gutmann–Beckett method](http://en.wikipedia.org/wiki/Gutmann%E2%80%93Beckett_method) by observing the phosphorous NMR spectrum of triethylphosphine oxide as it interacts with a sample lewis acid. 


An Acceptor Number (AN) scale is formed with AN=0 corresponding to triethylphosphine oxide in hexane (P chemical shift of 41), and AN = 2.21 (d-41), where d is the chemical shift of triethylphosphine oxide interacting with the sample. The greater AN is, the more acidic the compound. 


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