Quality - Calculation of Average

Hello

In quality I want to calculate the average

i have setup a plan with different colums

eg I am producing paper rolls, for the withd I weight different positions onto a sheet   lets say left right and middle of my sample

               L                      M               R                           avg

lot1

20

30

30

26,66667

lot 2         20                   null                30                       25

how would I calculate this in Quality (the tricky thing is that I have a null value ... which I do not want to consider in my calculation??

and for the same, how can I calculate the standard deviation per row (with different columns)

Thanks,

EJ