See
http://www.televue.com/engine/TV3b_page.asp?id=214
The 31mm Nagler effective field stop is 0.38" (9.65mm) above the reference line (base of eyepiece at top of barrel, see diagram on Tele Vue site). Refer column F. (F = -0.38 for 31mm Nagler).
Do you have any other Tele Vue eyepieces? If you have say the 17mm Nagler (F=-0.03) then you would have to rack in -0.03 - -0.38 = +0.35 or 8.89mm from where that eyepiece focusses. If you use a 1.25" eyepiece to compare with you also need to take into account the thickness of the 2"-1.25" adaptor you are using.
Find your focal plane by placing a sheet of paper over the 2" opening (no 2"-1.25 adaptor in place and no eyepiece) and focus on the Moon, (try tomorrow morning pre dawn, look at Mars/Jupiter too!). The 31mm Nagler needs to be racked in 0.38" from this position. If you don't have this distance available to you you need a lower profile focusser or you could try and loosen the collimation screws all the way and then recollimate; you may just have the 0.38" available.