How to manipulate the stimulus position precisely for online experiment

I’m trying to manipulate the position for each stimulus precisely on the builder for an online experiment using the units of height rather than in pixels. Because if I specify the location of my stimulus in pixels, it might display in the wrong place due to different pixel size across monitors for the online experiment. But the location of target stimuli is obtained in pixel, I wonder what will be the best way to make sure the size constant and position precisely for the online experiment.

Here is the sketch of the paradigm and parameters that I am using now. The participant will view a target object embedded in the stimulus (e.g., an image) following a centre fixation cross at the position [0,0]. I wish to display the target object on the [0,0] which following the position of the precedent fixation cross. However, the location of the target object varies across stimuli. For example, the size of the stimuli is 460 x 670, and the target object located at 230x 270. In order to display the target stimulus on the [0,0], I try to set the position (0, 300), for moving vertically at the moment and set the size (460,670) in the unit of the pixel to make sure the size will be constantly for the online experiment.

It is probably not making sense when I specify the location in pixel, and when my target stimulus is not well match to the location of the fixation cross…

I would greatly appreciate it if you kindly give me some feedback.

Thanks a lot!

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If you use height units then 0,0 is the centre of the screen. If you have an object of original size (460,670) then give it a new size based on how large you want it in height units, e.g. [0.343,0.5] so the aspect ratio is correct.

Thanks for your reply, I assume that [0.343,0.5] is for size in height units? So far If I understand well, there has no solution yet to convert directly from pixels to height see this post. At the moment, my solution is to set the size as (0.69,1) as the aspect ratio of the stimulus is 3:4. Then adjusting the target position to match the previous fixation cross of [0,0] by doing trial and error until it looks acceptable on my computer. It is probably not the most elegant and systematic way to handle it.