[mbar84]

If you receive btc in a transaction, the question is, when you consider it to be settled. In other words, how much energy is enough for you to consider it infeasible for an attacker to rewrite recent blocks so that you no longer are the recipient of some btc.

One way to think about it is to relate the total fees to the value of your transaction. If fees in blocks with a block height above the block with your transaction total $10000 (the security budget) then an attacker might be willing to spend that amount on energy to rewrite the chain. Another way to think about it is to relate the security budget to all recent transactions, assuming the attacker is the counter party to all of them (worst case).

In either case, there is no obviously correct answer to how high the security budget has to be on a per-block basis. The question is how long you're willing to wait for the security budget to accumulate and cover your transaction. If the block subsidy decreases and fees don't rise to replace them, then settlement time increases. Don't hand out goods to a counter party if the value of your transaction hasn't at least been met by the security budget. No need to wait longer than the point at which the value of all transactions of the block which includes your transaction has been exceeded by the security budget. An attacker would be losing money at that point if they tried to scam you.