You can make the CSS coding semi-automatic with CSS variables but this cannot handle percentages as used in the @kayframes statement.
Instead, you can calculate the percentages by hand (calculator) or I have also used MS Excel.
Doing so loses the closer understanding of what is happening.

Here is another demo: CSS - Simple Fading Clock Face.xar.

You could use the lower left CSS and the calculated @keyframes together to allow you to have any number of images.

Acorn