[Elecraft] Display frequency calibration

[Jack Berry]

I have s/n 3012, factory built. On a few occassions I have been told by a QSO partner that I am tuned a bit high. It has happened infrequently but I decided to check against WWV.
 
Three questions: 
1) is comparing to WWV a good method to confirm display accuracy? I used my P3 display to center the carrier at 2.5, 5.0, 10.0, 15.0 mHz. The centered carrier spike showed these offsets per WWV transmission; 
2.500mHz centered at       2.499.962
5.000mHz centered at       4.999.972
10.000mHz centered at     9.999.984
15.000mHz centered at   14.999.995
 
2) If there is a better reference outside a lab, what is it? 
 
3) And what is the procedure for correcting a frequency display error? I can't seem to find it in any of the manuals I have.
 
Thanks!Jack - WE5ST