[atrevbot]

When we were building our house I built a tool[0] for exactly this problem so I could visualize how tiles would look w/ different arranging. Worked really well for both floor tile and tile on the shower walls

[0] https://tilelayoutwizard.com/

[telcal]

I was looking for something like this when redoing my bathroom tiles but surprisingly couldn't find anything. I ended up using draw.io which worked but not that easily.

A few suggestions for your tool. Add millimeters as an option. I'm using 12"x24" tile for the floor but they're really 300mm x 600mm. I learned most 12"x24" are that size. Another thing I learned is with that size it's recommended to use a 33% offset pattern. This guy explains it well https://www.diytileguy.com/12x24-tile/