In this thread(French) Emmanuel mentions that it is possible to use a mathematical formula in numerical input fields.
I think it's worth mentioning in a separate topic so a larger group of users gets to know this.
What you can do is enter a formula like "100+3" or "((2*12.5)/3)+22" and the result will be set as the value.
Thanks Keet for the recall of this feature which appeared in version 6.0.
As explained in Tips section of the help, you may even use the math functions LN, LOG, EXP, SQR, SQRT, COS, SIN, TAN, ARCCOS, ARCSIN, ARCTAN.