No matter what dimensions and dpi I use in Illustrator when I drop the image in PPT slidemaster title background its never a perfect fit. Is there a perfect fit size? Ive tried

1024 x 768 pixels
1280 x 960

72 dpi, 96, 150....

and a bunch of other proposed dimensions from many posts. They all insert into a 4:3 slidemaster with a little extra whitespace on the bottom, requiring me to abandon the background file option and insert the file, then drag to correct size.

Just bugging me, is there correct 4:3 size or is it based on my monitor?