T-SQL (SS2K8) :: Replace Multiple Characters Based On Table Values

There is a table [Formula_Calc] with formula calculations that need to be replaced with relevant values based on another table [Totals]

[Totals]
RowNo|Total
F1|240
F2|160
F3|180
F11|1000
F12|1500
F13|2000

For example we've got a row from [Formula_Calc] table 'F1+F3' as a string that needs to be transformed as 240+160=400

The below code works for the above example but if I pick 'F11+F3' instead , returns 2561 which comes from 2401+16.
Probably replaces F1 value instead of F11 and adds 1st digit (1) if I got it right ...

DECLARE @formula NVARCHAR(100);
DECLARE @Total NVARCHAR(100);
SET @formula = 'F11+F3';

SELECT @formula = REPLACE(@formula,RowNo,Total)
FROM [Totals]

SET @Total='select '+@formula

EXECUTE sp_executesql @Total;
PRINT @Total;