[bokkoman]

On Kodi 20 has been improved some aspects for position settings,
there is "Manual" position setting to have a fixed subtitle position customizable by VIdeo calibration setting

or else

you can customize the subtitle vertical margin by using

Code:

subtitleverticalmargin

(see advancedsettings xml wiki)
allow you to change the default vertical margin (in px) used in all subtitle position settings like "Bottom of video"

We don't want a fixed postition. We are looking for a relative position, at least I am looking for that solution.

For example, 10% from the bottom of the video. For an 1920x1080p movie, that would mean it will automatically adjust it to 108px from the bottom. For 1920x800 movie, it will result in 80 pixels from bottom.

[Klojum]

We don't want a fixed postition. We are looking for a relative position, at least I am looking for that solution.

I already wondered who "we" was, hehe... But a relative position via percentages never works out well IMO when you have videos with different heights: playing a 21:9 video on a 16:9 screen has its challenges. I prefer to shift all my videos up vertically to the top of the screen, so that a black bar at the bottom appears. There I want my subtitles to be displayed, so they can be shown unobstructed. And on that 21:9 video occasion, I want the subtitles to be right under the video, and not at the bottom of the screen. The same with plenty of videos with a 'Netflix' screen resolution, such as 18:9.

In your relative approach, subtitles would be positioned at the different distance with every video height. Those differences would perhaps only be a couple of pixels, but it would still be noticeable.