Declaring a value in a formula field

[PETEHO]

If have an IF statement which generally returns a value however on occasions there will be no value to return to my formula field. How do I pre populate my formula field with the value of zero before the IF statement is run or can I put a zero in at the end. Sample code below

IF {?Period Number} = 1
Then {'B105_Nlforcm_'.base_amount01}
Else
IF {?Period Number} = 2
Then {'B105_Nlforcm_'.base_amount02}
Else
IF {?Period Number} = 3
Then {'B105_Nlforcm_'.base_amount03}
Else

Thanks

Pete