12.9 iPad Pro has an odd aspect ratio of 1.33 compared to 1.5 ratio of the image sensors of typical DSLRs.
This means that we cannot use the full real estate of the screen retaining the original full frame size.
To use the full screen resolution
1. Do all the image process (color/brightness)
2. Open the crop tool in Photoshop. Set the width and height on top of the crop tool (or height based on image orientation) to 2372X2048
3. Recompose the image using the crop tool. You may increase/decrease the crop rectangle like you do without setting width and height as mentioned in Step 2
5. Sharpen the image
It is better to keep the resolution at 275 ppi. Honestly this does matter for display devices whether it is 72ppi or 275 ppi. For print this does matter which is altogether a different topic