(1) I can cut 3" (or 7.6cm) and I can still reach focus with all my existing barlows and eyepiece combinations without using any extension tube
(2) I can cut 2cm more if I forget about using my 3x barlows.
(3) My binoviewer has an optical path length of ~12cm, so if I were to use it without any corrector, I will have to cut ~12cm or a little bit less due to the fact that the focuser is not racked all the way inside for the eyepiece with most in-travel.
(4) I have two extension tubes on hand, one is 1.5" and the other one is nearly 3" long, but I would want to use only one of them and I prefer that 1.5" since shorter means better.
Therefore, I can cut 4.5"... that translates to 11.43cm and it's just a little bit less than the required 12cm. Real life situation always ends up like this. LOL