You will have to fix the values of today's calculation before you make
changes to A1 or A2. I think the following formula should be entered
into B1:

=$A$1 * $A$2

and format the cell how you would like it. Then, when you have entered
today's values of A1 and A2, copy the formula into C1 ready for
tomorrow, then select B1 and click <copy, then Edit | Paste Special |
Values (check) and OK followed by <Esc or <Enter. The value in B1 is
now fixed, and the same value will be shown in C1. Tomorrow you can
change A1 or A2 and C1 will also change - copy this to D1 then fix the
values in C1, etc.

Hope this helps.

Pete