2) It would be nice to be able to set a ratio ( e.g. 16:9 ) when doing captures and cropping.
That depends on what you’re doing.
You have to type in values to get there, regardless of the goal.
With video, aspect ratios and screen resolution are fairly cut and dry. Resolution is based on 72 dots/pixels per inch. So a chart that you can Google will give you the numbers you need for video capture.
Images are not as cut and dry.
However, it’s probably best to keep it simple. A Photographic conversion chart would have to account for your monitors actual dpi’s to do you any good. I don’t know if anyone makes charts like that. Monitors dpi’s vary based on screen size. This calculator can tell you what your monitors dpi density is.It won’t help you change aspect ratios. http://kingscalculator.com/en/other-calculators/pixel-density-calculator
So, this pretty much leaves you stuck with the math. If you want a 3 x 5 photograph. You can’t multiply 3 and 5 by equal amounts. 30 x 50, 300 x 500, etc. And come up with a photo that’s 3 inches by 5 inches. The aspect ratio is right but the pixel density can be extremely low. Based on selection made “Actual size” and screen dpi.
As long as the aspect ratio is right, you can resize it by inches in SnagIt. But resizing can soften the image significantly.If you didn't capture enough pixels initially.
Maybe someone else knows a better way?