You may well get a more appropriate answer than this but here goes.
Make your photoshop image size the actual size you will print it and then 'fit to screen' so you can see it all. The maximum size of your image will be the size of your screens minus 1" at the sides and 2" either end - that's as big as you can print. Pull in the guidelines in Photoshop to split the image up into one A3 Portrait and then one Landscape across the top to overlap it. Position registration marks so that the screen across the top aligns with the portrait screen. When you print - don't print to fit and don't centre. Move the part you want to print into view. Print the A3 acetate across the top and then print the acetate for the portrait etc. If you are using more colours then you need to make sure everything registers.
Make your photoshop image size the actual size you will print it and then 'fit to screen' so you can see it all. The maximum size of your image will be the size of your screens minus 1" at the sides and 2" either end - that's as big as you can print. Pull in the guidelines in Photoshop to split the image up into one A3 Portrait and then one Landscape across the top to overlap it. Position registration marks so that the screen across the top aligns with the portrait screen. When you print - don't print to fit and don't centre. Move the part you want to print into view. Print the A3 acetate across the top and then print the acetate for the portrait etc. If you are using more colours then you need to make sure everything registers.
