This would best be asked in the Office Forum,
but here goes anyway:
You can't have the cell with the drop-down list (data validation) also hold a formula, so your formula must be in another cell.
Total in Cell A1
Drop down list in cell B1
Formula in cell C1
Create your validation list with multiply values in the next column:
1 Drop-down text Value
2 25% of total 0.25
3 50% of total 0.5
4 75% of total 0.75
5 100% of total 1
In cell C1, the formula will lookup the value in the table and return the corresponding value from the next column. The VLOOKUP function will do this.
The formula is:
When you select, say 25% of total in cell B1, this formula will lookup the 25% message in column G2 to G5 and return the value in the second column of the table, returning 0.25 which is multiplied by the Total in cell A1.
To stop this returning an error if you don't select anything in cell B1, wrap the function in an IF(ISERROR() function:
This only returns a calculated result if there is no error.
As A1 * blank returns an error, this formula returns a blank instead.
Here is the 25% result for a 'total' of 88.4 in cell A1
A B C
1 88.4 25% of total 22.1