I'll explain my own methodology, which has worked relatively well so far.

Usually, we have an idea what the size of the data store is (both at the 
beginning and its 'short-term' maximum).  Based on that, and an idea what the 
compression factor is (from prior experience), I run a series of calculations.  
Note that these are for a weekly removal offsite on a standard Full/Inc cycle, 
and can be modified for daily/monthly/etc as needed)...

-----=====-----
Weekly Data = (Total Gig [Full]) * 1.30   ## 30% added for Inc data
Tape Size = (200 Gig [LTO2] * expected compression)  ## 1.0 - 2.5+

[Optional] Tape Size = Tape Size * 1.?? (growth rate built-in, I usually put 
30%)

Local Tapes Total = (Weekly Data / Tape Size) * #-Wks
Clone Tapes/Week = round_up(Weekly Data / Tape Size)
-----=====-----

Once the number of tapes each week is determined we can start shopping around 
for media prices.  I end up 'quoting' tape costs by the number of boxes needed 
(LTO-2 is done in 20-packs) and bumping it to the appropriate dept for the 
actual costs.

I've actually generated some rather elaborate spreadsheets that take the above 
calculations into account, and allow me to play with different options and show 
what the tape costs are as the data size rises.  It's not as useful as it used 
to be (since it mostly calculates cloning tapes now), but has still held up 
well.

Our compression rates at times barely go above the tape capacity, but that's 
usually for data that was already compressed at 5:1 or better anyway (which is 
better than anything native I've seen for LTO-2).  This is more important than 
ever since we use plain adv_file disk as our primary target, and our 
longer-term data (databases) are exclusively created this way.

--TSK

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