Final PMT in a Series that results in a Specific XIRR??

I'm not sure what you mean. Your file says that the 200000 in F2 is not equal to the 200447 computed by the formula given above. I am not sure what you are seeing here, but I am seeing rounding errors. With 200000 in F2, the 16.0% in G2, if you expand the number of decimal places in the number format, is more like 15.9598....%, not exactly 16.0000000000%. If you put the 200447 computed by our formula into F2, then you will find that the test XIRR is much closer to 16.000000000...% (I get 15.99996...%). If you expand the 200447 and enter the value to its full precision (or copy->paste special -> as values) you should see that the XIRR computed in G2 is essentially 16.0% -- to match the 16% entered into A2.

I could be misunderstanding, but, if I am understanding, this looks like a case where you are not looking at the computed numbers to their full precision.