You can right click on any size in the print size box. That gives you a complete menu of what you can do.. i.e  make a new size, remove it, save is as a permanent size, etc.

I don't quite understand the purpose fully.
... to aid in trimming with rotary trimmer.

I can understand wanting an extra .125 of white space to have room for error when trimming, but making the print size larger by .125 doesn't seem to satisfy making it easier to trim.
The edge of the print is still the same.

and simple way of selecting priority areas of the image for exposure.

If you take the time to learn QU, and being a fair person, will honestly compare....
I will take any Raw image or even a JPG, and make a better final image and print using Qimage than can be popped out of PS.

Raw processing ... QU does it automatically to 90% of spot on.  

I found an old video for you
https://www.youtube.com/embed/AiVoXcB1uzk

My point... Ink price is little concern.
Paper cost little concern.
This is a hobby for us. We take pictures and catch good light on an interesting subject, and it starts there.

If this is your business, then like any business, you balance costs against results.

I think you have to decide to enjoy printing or give it up and let Walmart print for you.

Not trying to sound harsh, but there is no middle ground. 
If you bought a decent printer and do not use it, you will have clogged coagulated ink cartridges, and dried out head. Many many ink cleaning cycles.... to recover. THAT gets expensive.

Again, apologize for sounding harsh
Fred

The auto job log in QU allows you to see how much area has been prints with ink and the total area of the media used. I've used this data to do detailed costings of my printing.

do not try to "enlarge" the image before you print it. Let QU do the interpolation and use the original image at 10x8.