Forget memorizing the pace you need to maintain to calculate your marathon time.  Here is a foolproof guide to estimate your finish time.  This applies to race day assessment of yourself, how you feel upon reaching the halfway mark.

1. Wow, 13 miles DONE?
Go for it. You are on pace for a negative split.

2. Yey, 13 miles LEFT to go!
Nice control. You are on pace for an even split.  +/- 2 minutes your halfway time.

3. Wtf, 13 MORE miles to go?
Start praying. You’re way out of whatever goal you set.  +10 minutes or more to your halfway time.

Come back here and leave a comment after your marathon to prove or disprove this formula.