In chapter 6 of Keynes' General Theory, he briefly mentions Effective demand...
"Furthermore, the effective demand is simply the aggregate income (or proceeds) which the entrepreneurs expect to receive, inclusive of the incomes which they will hand on to other factors of production, from the amount of current employment which they decide to give. The aggregate demand function relates various hypothetical quantities of employment to the proceeds which their outputs are expected to yield; and the effective demand is the point on the aggregate demand function which becomes effective because, taken in conjunction with the conditions of supply, it corresponds to the level of employment which maximises the entrepreneur’s expectation of profit."
We see here that employment can be a constraint upon expectations of profit. Firms hire labor when they have expectations of profits. Aggregate demand is a function of the proceeds yielded from labor's work. Keynes is saying that there is a point where firms will not hire more labor because profits from labor will be maximized.
He calls that point effective demand.
OK... But he does not give an equation to determine the effective demand.
I will continue going through the chapters of Keynes' General Theory and making comments where effective demand is mentioned.