you should use ACTIVE_AREA parameters for width-mm and height-mm. In my case, I always round up the numbers - I have never seen the usage of decimal numbers in any display DTS overlay for these two “variables”.
hsync-len and vsync-len stand for an hsync and vsync pulse width. If the datasheet does not mention these parameters specifically, you should at least look for a picture like the one below
In the picture, “128 dot” stands for hsync-len and “4H” for vsync-len. I presume that the description may be different in your case so use this just as an example.
Thanks for reaching out to us and also sorry for the delay in answering.
From the datasheet of your display, hsync-len should be the HORIZONTAL_BLANKING_PERIOD (which should be 88) and vsync-len should be the VERTICAL_BLANKING_PERIOD (which should be 22).
Here is a good read about adding custom displays: Adding a custom display
I hope that helps. Please let me know if this works for you.