For example: Client sells stock in October and makes a long term profit of $100,000 on top of their regular salary of $60,000. I enter Total Income for 3 months = 15,000 and capital gain for the period as -1. Total income for 5 months = 25,000 and capital gain for the period as -1. Total income for 8 months = 40,000 and capital gain for the period as -1. That probably leaves the client only owing underpayment penalty for the 4th quarter estimate due 1/15 of the following year.
If the gain were in July, the only change to the above would be: Total income for 8 months = 140,000 and capital gain for the period as 100,000. That means penalty for 3rd and 4th quarter payments.

What is your "offending item?" i.e. what triggered the underpayment and when did it happen? I use capital gain, as that is what I saw most often. Bonus salary can be another, in which case you would need some pay stubs for the end of each quarter. I only use my cheat as way to not deal with minutia, when it does not significantly impact the difference in the actual amount. Ideally you could track down every penny, but would it make a large enough change to warrant the cost to the client compared to what the 2210 penalty is.