i ned help on control saource

hey i am having difficulties
basically i have 4 frilds named:
1)sub total-( this is the total not including the vat)
2) VAT-( this is wther the vat is 0.00 or 0.175) 0.175 being the vat rate
we
use in the UK.. for this feild i created a drop down list which consisters
of
:
0.00
0.175
3) VAT (T1)- Vat (t1) is the sum of sub total * VAT 
4) Total including VAT- (this is the sum of sub total + VAT(T1)

ok in the control source for VAT T1 i entered the following as my formula
:
=[sub total] * [Vat]
for my total including vat feild i entered this formula:
=[sub total] + [VAT (T1)]

now in my forms these formulas seem too be working perfectly fine
however wen i go bak to my table ... both VAT (T1) and TOTAL INCLUDING VAT
come up as 0.00 :S:S
maybe i am writing the formula wrong? also for the feild VAT it is a drop
down box so am not sure .. if this changes anything
please could anyone kindly get bak to me as i am indeed struglling 
regards Noor