Paul,
if the only options fo Reintype(E12) are blank, "Assumed" and "Ceded"
then the following should do.

=IF($E12="","", VLOOKUP($B$6&G12,[Data.xls]DECAP*_ALL2!A:W,
IF(Reintype($E12)="Assumed", 15, 17+($I12-1)), FALSE))

HTH
Kostis Vezerides