Would anybody else here find it really useful and care to comment on it too? It's version 1.0.0 and so open to suggestions for improvements by real life users.
Sorry, but I think it is entirely pointless. Details are getting in the way of the point.
Generally we want aspect ratio to fit the size of the paper we are printing the image on. We generally try to print at 300 dpi. To print 8x10, we do want 2400x3000 pixels. To print 4x6 is 1200x1800 pixels. But it requires a human brain to LOOK at the image, and decide the best way to adjust the crop box. If you don't care, and just send it for printing anyway, the automatic machine will make it fit the paper, perhaps with a few heads missing or something, but 6x4 will always come out 6x4.
The point is too LOOK first.
If we use Adobe software (Elements, Lightroom, Photoshop, etc), we already have easy tools to make this be a trivial operation (perhaps a few others have it too?)
Simply set the marquee selection box to the ratio 3:2. Then any crop box you mark, of any size, will be 3:2 shape. Crop it how you want it to look. Then resample it to 1800x1200 pixels (or resample to 4x6 inches at 300 dpi).
Cropping 4:3 to print 2:3 is trivial. Just add the crop box at 2:3, and slide it up or down to better center the subject.
Cropping 3:2 to print 6x4 is trivial. Just add the crop box at 2:3, and slide it towards either end to better center the subject.
You may want to make the crop even a bit smaller, to improve composition, remove blank border areas, etc.
Then resample to 1800x1200. Often this is a batch operation. In Photoshop Scripts, you specify Fit To 1800x1800, and then either portrait or landscape oriented images come out perfect at 1800x1200, or 1200x1800. Couldn't be easier.
There is also another Crop tool too, similar operation, but I find the standard marquee box easier. If not careful, some smaller cases (already smaller) will allow enlargement, in which case it would be better to print the pixels you have at a lower resolution.
Image Resize - Cropping, Resampling, Scaling describes this.