If my calculations are correct you will get 85% of the age 60 amount of $3,500 per year or $2,959. Since this will begin I presume to be cola adjusted once you start the amount you are short will be determined by the inflation rate.
However, I say grab the $250 a month. Assuming a 3 real return and 2% inflation and a 35% tax rate you will have an age 60 pension of $3,266 and $13,000 in the bank which would amortize your shortfall of $234 per year for an excess of 30 years.
But then what do I really know?