I have seen so many troubles because people got the border wrong. It should be a very common sense kind of thing, but many complicate it beyond what is needed.
First, draw the border full size in Model Space. If it is to be 24″ x 36″, then draw the trim lines exactly that size. Use ORTHO lock to make them exact. Use the Rectangle command to get them right.
When you are done, plot the file from model space at 1:1. Check the file with your scale on the hardcopy to verify that it is exact.
Save the file.
Now Xref the border file onto your sheet.
The Border is Xrefed into Paper Space of your Sheet files.
In the olden days it use to go in Model Space. Back in the “stone age”. You know, before Paper Space was invented. Do not place the border in Model Space. It now goes in Paper Space.
Attach the file at 0,0,0. Use a layer that you create – Not layer “0” or “defpoints”. You did not create or own these layers. They belong to Autodesk, not you.
Do not scale the border when you attach it, larger or smaller. Keep it at 1 to 1. That is full size. One inch on the border will be one inch on hard copy and when measured in CAD.
LTSCALE should be set to “1” for plotting on all Sheet files.
Some simple guidelines, but often not followed.