What is the legal test for "second guessing" competent counsel?

It is a fine line to be drawn between “second guessing” competent counsel and determining that counsel acted with excessive zeal or extravagance in incurring the impugned disbursements. On that question, I do not understand Master Horn’s comments in MacKenzie v. Darke to mean that the registrar must defer to counsel’s judgment on all matters and essentially issue a carte blanche to incur disbursements at the ultimate cost to the defendants. Such an approach goes against all of the well-established authorities and makes the assessment process meaningless. Master Horn’s comments ought to be considered in their context. The actual retention of an expert was not in issue in that assessment. The issue was whether it was necessary or proper to retain a new expert or stay with an expert who already provided an opinion with the latter choice being more costly at the end of the day.