I'm trying to create a formula that applies different factors and then
promotional factors to a base rate. This was working perfectly with IF
functions until I had more than seven possible promotions. My previous
formula was:

=IF((B14="BOGOF"),($A$15*B12)*$Q$4,IF((B14="PRICE" ),($A$15*B12)*$Q$5,($A$15*B12)))

(So my promotions are called BOGOF, PRICE etc. (there are now 14 different
options) and have different factors to multiply the base rate ($A$15) and
then regardless are always multiplied by the seasonal factor (B12).

What I don't understand, now needing to change to some kind of LOOKUP, is
what happens if there is nothing the promotional line - every example I can
find seems to enter a blank box or N/A or FALSE whereas I need it to fall
back to the end of the above formula if there is no promotion on: ($A$15*AB12)

I tried combining IF and ISTEXT but couldn't get them to nest.

Any help would be very gratefully received! Thank you.