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Peter, the focal length of your mirror is the distance from the centre of the mirror to the point where light bouncing off the mirror comes to focus.
If the secondary wasn't in the way, that cone of light would focus 1800mm above the primary.
The secondary's job is only to bend that cone of light to the side so that we can view the image without getting in the way of the incoming light path.
The 'intercept' is the distance from the centre of the secondary to where the focal point is. The exact distance of the focal point above the racked in focuser
varies depending on scope designs. On my Dob it is only 11mm above but most of the SW/Orion/Bintel/GSO scopes have it about 20-30mm .
Adding the intercept distance to the distance between the primary and secondary will give you the focal length.
If you want to see where the focal point is on your scope you can simply place some translucent tape across the focuser tube (NO EYEPIECE) then point the scope at the moon and bring the image of the moon as sharp as possible on the tape by adjusting the focuser outwards. That will be where the focal point is.
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