Look at the OAG adapter and locate the position of the prism on it. Measure how far its diagonal surface is away from the main camera's sensor, taking into account the flange to sensor distance of the main camera (for ZWO cameras, it is usually either 12.5mm or 17.5mm following Cs-mount and C-mount flange distances).
Next, check the distance from the diagonal surface of the prism to your guide camera's sensor, again taking care of the flange to sensor distance of the guide camera.
Set up the guide camera's focuser so that the distance from the prism to the guide camera is precisely the same as the fixed distance of the prism to the main camera's sensor. This should get you to within 1mm of the correct focus location. If the prism were located on the optical axis, then the distance from the prism to each camera sensor should be precisely the same.
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