John, that's spot on - thank you! Had tried the same but think came unstuck with the control-shift-enter and lost confidence in the ability of OFFSET & COLUMN to replace OFFSET & ROW for Horizontal data.

I did by the way come up with this little one to deal with the problem if i couldn't find a solution - benefits of this approach is it doesn't use a "volatile" function, whilst it is limited in that it gets capped at 7 days.
'=IF(B3-SUM(C2:INDEX(C2:J2,7))>=0,">7 Days",IF(B3-SUM(C2:INDEX(C2:J2,6))>=0,6+(B3-SUM(C2:INDEX(C2:J2,6)))/INDEX(C2:J2,6+1),IF(B3-SUM(C2:INDEX(C2:J2,5))>=0,5+(B3-SUM(C2:INDEX(C2:J2,5)))/INDEX(C2:J2,5+1),IF(B3-SUM(C2:INDEX(C2:J2,4))>=0,4+(B3-SUM(C2:INDEX(C2:J2,4)))/INDEX(C2:J2,4+1),IF(B3-SUM(C2:INDEX(C2:J2,3))>=0,3+(B3-SUM(C2:INDEX(C2:J2,3)))/INDEX(C2:J2,3+1),IF(B3-SUM(C2:INDEX(C2:J2,2))>=0,2+(B3-SUM(C2:INDEX(C2:J2,2)))/INDEX(C2:J2,2+1),IF(B3-SUM(C2:INDEX(C2:J2,1))>=0,1,B3/C2)))))))

Where Sales Starts in B2, and stock in b3 (sorry new to this so couldnt upload a file)