Mosaics

Just trying to experiment by trying to do a mosaic, but can’t get the contact sheet layer right. I’ve set the page size to 10inch x 8 inch and have 1280 pics at 1/4inch square.
With a setting of 40 columns and 32 rows with 0 gap, I get an error message
of "the values you have entered for document width or height, thumbnail columns or rows produces an invalid thumbnail width".
An I missing something here? do I have to allow for something?
If I set for auto spacing I get a small gap between thumbnails.
Any help would be appreciated.
Alan

Just trying to experiment by trying to do a mosaic, but can’t get the contact sheet layer right. I’ve set the page size to 10inch x 8 inch and have 1280 pics at 1/4inch square.
With a setting of 40 columns and 32 rows with 0 gap, I get an error message of "the values you have entered for document width or height, thumbnail columns or rows produces an invalid thumbnail width".
An I missing something here? do I have to allow for something?

If I set for auto spacing I get a small gap between thumbnails.
Any help would be appreciated.
Alan