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Peter,
It's your #2 the distance from the mirror surface to the focus at the eyepiece.
If it says it's 1800mm, this is probably close enough.
The f ratio is FL/ mirror diam = 1800/ 400 = 4.5
(you can double check the FL - at the next bright moon remove eyepieces and focus the moon's image on a card. measure the distance to the focuser, then the focuser to the middle of the secondary and finally secondary to mirror ; add these numbers and you have the FL)
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