picking up right filed

[H]

hi

i have a table with 4  fields

field 1 =  number result 1

field 2 =number result 2

field 3= percentage result

field 4 = calculation by taking in whichever is lower field 1 or 2 and multiplying by field 3

 

the question now is how do i script the calculation it should pick up which ever is lower but ignore if field is empty.

i hope i am clear , i would appreciate if someone could help me out with this calculation. 

 

[Ocean West]

Min ( Field1 ; Field2 )  *  Field3