I have a need to calculate a formula that I don't know how to
reference. The following is the pattern that I could hard code.
Basically, I have a value for each year, and need to interpolate
monthly values. Is there a more generic way of entering these
cells?

=D1 + (D15-D1)*1
=D1 + (D15-D1)*2
=D1 + (D15-D1)*3
=D1 + (D15-D1)*4
=D1 + (D15-D1)*5
=D1 + (D15-D1)*6
=D1 + (D15-D1)*7
=D1 + (D15-D1)*8
=D1 + (D15-D1)*9
=D1 + (D15-D1)*10
=D1 + (D15-D1)*11
=D1 + (D15-D1)*12


=D30 + (D30-D15)*1
=D30 + (D30-D15)*2
=D30 + (D30-D15)*3
=D30 + (D30-D15)*4
=D30 + (D30-D15)*5
=D30 + (D30-D15)*6
=D30 + (D30-D15)*7
=D30 + (D30-D15)*8
=D30 + (D30-D15)*9
=D30 + (D30-D15)*10
=D30 + (D30-D15)*11
=D30 + (D30-D15)*12


--
"In no part of the constitution is more wisdom to be found,
than in the clause which confides the question of war or peace
to the legislature, and not to the executive department."

- James Madison