"Crackles McFarly" wrote...
....
OK, here is my example I am using

Initial investment of $500.00
With $100 per month, every month.
At a return rate of exactly 10%
For a period of exactly 25 years.

Is your 10% return an effective annual rate or nominal compounded monthly?
Are your additional investments at the beginning or end of each month? I'll
assume at the end of every month.

First assuming 10% nominal compounded monthly.

FV: =FV(10%/12,300,-100,-500) returns 138,711.81.

Next assuming 10% effective annual.

FV: =FV(NOMINAL(10%,12)/12,300,-100,-500) returns 128,749.84.