regarding addition of new column in existing appraisal templates

hi gurus,

i m working on the existing appraisal templates in which i have to add one more column named "Avereage Rating"

in which the average of "MID YEAR ACHIEVEMENT COLUMN TOTAL VALUE" & "FINAL APPRAISAL RATING COLUMN TOTAL VALUE"

for eg:"(3.5+4.0)/2 = 3.75" i.e the value 3.75 will come in the average rating column,plz help me to sort out

this column,for this,no doubt i have added the column for this but for the value "3.75" what "value list",

"value class" & "value determination" sud i use against this column plz help me....