The Approximation Formula. F = Face Value = Par Value = $1,000P = Bond PriceC = the semi annual coupon interestN = number of semi-annual periods left to maturity. Example. Find the yield-to-maturity of a 5 year 6% coupon bond that is currently priced at $850. (Always assume the coupon interest is paid semi-annually.)Therefore there is coupon interest of $30 paid semi-annuallyThere are 10 semi-annual periods left until maturity .