HLOOKUP - Relative cell reference in row index number

Hi. I'm using an HLOOKUP where the row index number is a formula which refers
to a cell in the same row as the lookup function. This is then applied to the
whole column. It seems that, whether or not I use $, the cell references in
the formula are absolute which means that when I sort the worksheet by
another column the HLOOKUP uses the wrong cell reference. Is there any way
around this?

The whole lookup function is the following:
=HLOOKUP(I3,'Fee
matrix1'!$1:$857,(2+(IF(Data!C13119,Data!C131,1)) +Data!K131*19),FALSE)

so the formula for the row index number is
(2+(IF(Data!C13119,Data!C131,1))+Data!K131*19)

Thanks in advance for any help you can give!