Operation by task not by hours

In some industries, the cost of labour is not givin in “hours of labours” for the operation, instead, they used “labour by item” cost, and the hourly cost could change or be variable, since not all operators or workers have the same productivity rate.

Example: in construction industry, the labour for installing floor or painting a wall, is given in cost per square meter, instead of cost per hour, so an skilled worker make more square meters by journal than less skilled workers.

Anyways, the cost of installing floor, or painting, depends on square meters to be covered and there’s no hourly rate for that.

How can be change the operations in the BOM or in Task to “per item” or “per square meter” instead oof “per hour”.