It is normal to have border regions in every edge of screen and different programs can use different screen sizes. There is no "standard".
And as was replied previously, there are many "PAL" Amiga games that only use NTSC height and by design have big black bar at the bottom. (and run in wrong speed in NTSC mode)
Only exception is few full overscan programs (for example The Settlers) that fill the whole screen but they also can have slightly different horizontal positioning.
-> You can't have single perfect monitor (or emulator) size/position adjustment. Amiga is different (too flexible
) compared to most other computers or PC VGA etc.. (8-bit Ataris is one exception but it also does not have as flexible screen width configuration)