Simon Tatham has a completely guess free minesweeper in his portable puzzle collection: https://git.tartarus.org/?p=simon/puzzles.git;a=blob;f=mines...

The basic idea is he generates a grid, runs it through a deductive solver, and if it fails, heuristically changes the grid a little where it failed and tries again.

I'll try some different ratios and get back to you!

edit: I tried some runs with size 3 and 20% mines, that felt a lot more like expert difficulty Minesweeper too in terms of the sizes of the empty patches you uncover by uncovering a 0.