I have a two part question to solve and I hope what is written makes sense

1) in the F column - F1 =12 unless A1 starts with either GM or STW
this is to fix a calculation later i would then fill down to cover the F
column

2) the following is in G column but I need to add "ignore calculation if D1
is blank" then i would fill down to cover the rest of D
=IF(ISNA(VLOOKUP(B20,Sheet2!A:F,4,0)),"",VLOOKUP(B 20,Sheet2!A:F,4,0))

Many thanks