>Does anyone have a mathematical formula for predicting tape usage for
>transaction logs?  Any idea on how to do this for something like this that
>has so many variables?  

Hmm, let's see. 

((X / (1024 ^3)) x Y) / Z) x W

Whereas:
 X = The number of bytes your db's generate for transactionlogs

     A nice estimation of X would be, the number of users connected 
     to your db x the number of keystrokes they do per day x their 
     intelligence factor divided by the coffee cocient, which is an index
     for the quality of the coffee your company serves. 
     0.1 is bad coffee. 1 is very good coffee. So 0 >= X <= 1
     
 Y = is your retention, expressed in days. 
 
 Z = is the capacity of one tape, expressed in GB's. 
 
 W = a factor expressing the quality of the backup admin. 
     W = 100 = Bad admin
     W = 1   = Good admin
     So, 1 >= W <= 100
     
Sorry, couldn't help it. Nothing better to do :-)
 
 
-- 
You're to have, not to hold.