Given the blog post is about 10 years old now some parts are out of date.
It's hard to give a specific number for image memory as it depends on the devices you want to run it on and the amount of memory used for other parts of the game. As a guess I'd say 500mb image memory is probably OK on most devices these days, but you should test it yourself.
To be honest I'm not sure to what extent the power-of-two rule applies in terms of memory usage. Most modern GPUs support non-power-of-two textures, but that could be achieved while internally placing it on a power-of-two texture and pretending it's a smaller non-power-of-two texture, so it depends on internal details of the GPU and driver. It doesn't matter much with spritesheeting though, as all spritesheets are power-of-two sized.
Construct's image memory measurement is for the entire project and includes the overhead of the backbuffer, which is related to the viewport/window size. So you can't use that to measure the memory usage of a single image. Even then the image memory usage is based on a simple width x height x pixel size calculation, and cannot account for the GPU changing the memory representation internally (e.g. with padding, or in some cases lossless compression).