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to illustrate the example, I did a quick mock up in TT based on an 18k safe harbor with the Q1-2 underpayment showing the penalty and Q1-2 overpayment showing no penalty, see below. so yes if you overpay earlier in the year and meet safe harbor by the end of the year you will not have a penalty.
At the end of the day it all depends how material any underpayment errors are - in the example above the penalty is $40; even tho the underpayment rate is 8% the underpayment is extinguished by the overpayment within a couple of quarters which stops the penalty from accruing.
re your calculation it sounds like you have a good handle on the salary+RSU whether it's X or Y they sound close; but if you leave interest/dividends/cap gains until the end and didn't include that in your estimation as you go, then you could end up with larger payment needed in Q4 and back to either a penalty or using Form 2210 AI method to reduce it. Again depends how material this is vs. X or Y. Personally I take a stab at estimating this for the year, include that in ES payments quarterly and then review later in the year.
