When you mix and match brands it might not always work, ie Takahashi, Baader and Televue.
The easiest and approximate way to work out is to use your focal length of your telescope.
Approximately measure where your front element meets the tube down to the back of your visual back (imaginary centre line). Now add on the diagonal which has a 112 mm light path. (I have the same one). Insert the eyepiece, measure to the front element and add the eye relief of the eyepiece.
If this is longer than your focal length of the telescope, you need to reduce it by at least that much with a shorter extension. Make sure your focuser is racked in all the way!
Do this with your longest and shortest eyepieces to make sure they will all come to focus and add a bit of leeway.
You can always rack your focuser outward from here, but not in!
Hope this helps
