The size problem of ‘deg’ units of grating stimulation

OS (e.g. Win10): Win 11
PsychoPy version (e.g. 1.84.x): 2024.2.4
Standard Standalone? (y/n) If not then what?:
What are you trying to achieve?: Display the grating stimulation in full screen using deg units
What did you try to make it work?: Use anothe units “degFlat”
What specifically went wrong when you tried that?:
It’s something that’s been bothering me for a long time.
I want to use deg units to build a full-screen grating stimulus on my monitor, but when I set the size in the grating properties, the grating image I get doesn’t match the size calculated using trigonometry!
My monitor info:
Resolution: 2560 x 1440pixels
Width: 61.13cm
Distance from screen: 30.565cm
If I want the image to spread over the full screen, the calculated w of size=2*arctan(30.565/61.13/2)=90°, right?
But when I set the size’s w to 90°, I get a grating image that is very small, I don’t know what’s wrong, can anyone help me? Thank you very much!
There are some picture of experimental setting:


I’m not sure if I successfully uploaded this topic. Anybody can help me?