What is the test for estoppel by acquiescence?

And at p.15: ...the application of the Ramsden v. Dyson L.R. 1 H.L. 129 principle -whether you call it proprietary estoppel, estoppel by acquiescence or estoppel by encouragement is really immaterial- requires a very much broader approach which is directed rather at ascertaining whether, in particular individual circumstances, it would be unconscionable for a party to be permitted to deny that which, knowingly, or unknowingly, he has allowed or encouraged another to assume to his detriment than to inquiring whether the circumstances can be fitted within the confines of some preconceived formula serving as a universal yardstick for every form of unconscionable behaviour.