I can't get excel to autofill my formula correctly. I need it to refer to
every sixth cell on another sheet. Excel just chooses the number of cells I
click for autofill and increments the cell number by that much.

e.g., =if(isnumber(sheet2!c1)... then next row down is =if(...c7) and next
is c13, etc. the autofill pattern for the next cells is c4, c10, and c16. I
need it to be c19, c25, and c31. Selecting 6 cells doesnt work either, i.e.,
1,7,13,19,25,31 produces 7,13,19,25,31,37. How do I go about this?

TIA