Here is our version of the Gray Card for determining correct exposure.
The suggested .pdf file is a 18 percent gray fill with additional white and black sectors.
All you have to do is download the file and print it out on your printer*.
Based on this "gray card" you can not only correctly determine the exposure, but also use it to color-correct the resulting image in a graphic editor (for this, of course, it is necessary to leave the printed sheet when you take the picture so that it is somewhere in the corner of your picture and you can just trim it later).
You can read how to use the "gray card" here.
Look for more free stuff here soon on how to use a gray card with a modern smartphone (yes, even with the iPhone 14 sometimes you can't do without a gray card).
(If you're into free homemade paper photo crafts - make a free lens hood).