Resizing Images for Printing and Mounting

This may well be wrong, because I worked it out myself......all comments and ideas welcome.

For the BPS competitions and for the Burton Hospital Exhibition we are using a maximum mount size of 50cm x 40cm.  Obviously images can be any size within that mount and any orientation, but this is just one way of resizing to optimize framing.

Say we have a portrait orientation image and we want our mount borders to be 7cm wide at the top and left and right sides and then 8cm at the bottom border, which justs lifts the image and works better optically.

In Photoshop select the crop tool and set the width to 27cm and the height to 36cm, this makes the image 1cm bigger than the cut out of the mount allowing 0.5cm overlap on each side. Set the Resolution to 300DPI 

To return to free cropping afterwards just select Clear.

Then I go to Image, Canvas Size and select inches as my units.

I select 12inches by 16inches as the Canvas size with an extension colour of white.
(16x12in is the same price as A3 at DS Colour Labs)

I'm then ready to Edit, Convert to colour profile which I've downloaded from their web site, this is DS Colour Frontier DP2 Lustre, for the matt finish. And obviously Save As ready to send by email.

For 6cm borders top, left, right and 7cm bottom,
Landscape crop 39cm wide by 28cm height
Portrait crop 29cm wide by 38cm height.

For 7cm borders top, left, right and 8cm bottom.
Landscape crop 37cm wide by 26cm height
Portrait crop 27cm wide by 36cm height