Assume there are values in A:A

- starting at A2 and ending somewhere in A:A

- with gaps between both (=empty cells)

- with a maximum gap of 98 cells,

use for interpolation along A:A the following to-be-filled-down array formula:

B2: {=B1+(INDEX(A:A,MIN(IF(A2:A101<>"",ROW(2:101))))-B1)*1/(MIN(IF(A2:A101<>"",ROW(2:101)))-ROW(A2)+1)}

A B 1 2 1 1 3 2.5 4 4 5 5.5 6 7 7 7 4.333 8 1.666 9 -1 -1

=IF(A2<>"",A2,aboveFormula) makes it the faster, the less gaps you have.

For a maximum gap of f.e. 998 adjust it to A2:A1001 and 2:1001, respectively.