When I did my initial calculations I just created an excel with some different possible Scenarios in terms of active stake assigned to the pool.
Find it here: Stakepool Profitability Calculation - #15 by zwirny
The main topic is that the 340ADA are only provided if you mint at least one block. This is not the case when you have a small pool where you will not mint blocks in most of the epochs and therefore also do not get any Operator Costs.