The bug's-eye view is the marginal approach. We can break that question down.
Profit is the difference of revenue minus cost.
The first question is relatively easy. What one additional labor unit will add to cost is the wage paid to recruit the one additional unit. The second question is a little trickier. But we need a measurement that is comparable with revenues and profits, that is, a measurement in money terms. The Value of the Marginal Product is the product of the marginal product times the price of output.