What is the legal test for calculating the reasonable hourly attorney fee?

The court determined that there were difficulties in trying to calculate the reasonable hourly rate according to a " 'cost-plus' " approach. Under that approach the fee would be based on " 'the sums paid out to [the] attorneys as personal income and to defray overhead costs attributable to the maintenance of the attorneys in the firm,' plus a 'reasonable and controllable margin for profit.' " (Copeland v. Marshall, supra, 641 F.2d at p. 896.)