Splitting high resolution in tiles, and saving them out as new cropped and layered psd file.

Hey there, my first post here.

I currently working with designing photos/images ceramic tiles.
My goal is to minimize the time it takes to split an image in equal tiles, and then having all layers put in a new layered psd-file and layers named after raws and columns.
Let say I have a bathroom wall 160X100 centimeters.
That would be 8x5 tiles, tiles are 20x20 cm I this case, 40 tiles.
Doing this manually when the work load increases would be impossible to manage.

Tried using slices, but there's gotta be a bug in relationship between pixels and millimeters/centimeters.

I divided the whole image in equally big slices 5 raws, 8 columns.
No matter what the pixels say, when the image is 1600x1000 mm, one would think the result would be 8 tiles X 5 tiles that are 200x200mm each!

But it turns out it differs 2 - 3 millimeters.

Would be glad if there was a different approach to this, either by a neat script or a plug in or something.

Images slicing in photoshop is only able to save out to web formats what I know of. I need it to be high resolution images 300dpi.

Hope th get some feedback and ideas on this.

Thanks in advance