In control variate technique we have to calculate
$$b=\frac{\text{cov}\{{X,Y}\}}{\text{var}\{{X}\}}$$
where $X$ is a payoff from standard call option and $Y$ is a payoff from for example barrier option. Why we have to estimate $b$ before we use this method and we cant use payoffs which we use during pricing? Or maybe I can first calculate the payoffs for these options, then based on them, calculate the option price, and finally calculate $b$ using the same payoffs and change the price of the barrier option accordingly?