P/B ratio

Okay so forecasting earnings using ROE the way is to multiply ROE with Book value of equity(B0) at the beginning of year or end of last year.

But ROE is Earnings/avg. value of equity(=(beg equity+end equity)/2) … So instead of ROE shouldnt the method be to use average En/Bn-1 over hte years to forecast the E1 using the B0 values??