A geometric series is not the proper tool for this. I think you’d need to use a stochastic model, it’s been years since I got my B.Sc in Computer science so I don’t have any links to back it up.
Why make things complicated when simplicity will do.
Premise #1:
I think we can all agree if you had 6 bombers on a raid, statistically speaking, you should lose 1 bomber.
Premise #2:
Each surviving bomber will do 3.5 IPC
Premise #3:
5*3.5=17.5
Conclusion
If you spent 15 IPC per turn for a bomber, started with 6 bombers, you on average will do 17.5IPC per round.
This is called a sound argument. If all the premises are true then the conclusion must also be true. Prove 1 of my premesis wrong and my conclusion is not logically true.
However it is statistics. Did you hear about the three statisticians who went on a hunting trip? They see a deer, the first statistician shoots and misses 5 feet to the left, the second statistician shoots and misses 5 feet to the right. The third statistician yells out “BULLSEYE!!!”……
The 15:17.5 payout ratio does not take into effect what those bombers could have done if they were used in a land or naval battle instead. They do not take into acount the cost of losing your only bomber. Losing your only bomber means your reach is less and your enemy has less to defend against. 1 bomber plays havoc with plans requiring a 1-2 transports to move through it’s area of reach.
I totally agree with Ozone, SBR are not a strategic, merely a usefull tactic in some situations when the opportunity or need presents itself. Using SBR has a strategy locks you into 1 mode of attack.
BB