Hi @rasa, I would say the easiest thing for you to do (and the quickest) is to first make an assumption on the distance between the user and the screen..lets say 30 centimetres. (this is nominal, but you could do some testing over a period of time to determine an average)
So, if we place the first (lower left) stimulus point at (100,100) then the next stimulus point along the x axis would be 20 degrees to the right..
To get this position, this can be calculated with simple geometry: Distance to next point along x-axis = tan(20 degrees) * 30 cm = 10.91 cm
We now convert this to pixels (https://www.pixelto.net/cm-to-px-converter) assuming a DPI of 96dpi: 10.91 cm = 412 Pixels
Therefore next stimulus points would be (100+412,100) = (512,100)
You then apply this similar technique to the rest of the points. Hopefully, this procedure is clear enough.
Please let us know if we can of any further assistance.