With the eyepieces to see M45, a good point to start is work out how big the target is (in this case about 1.4deg, or roughly 3 times the diameter of a full moon) and then use the calculator on the Televue site to work out a true field. (the address is here
http://televue.com/engine/TV3b_page....212&plain=TRUE )
With your scope the TV eps that do it are
35 Pan
41 Pan
31 T6 Nag
21 Ethos
55 Plossl
Of course the size of M45 is the entire cluster, not just the central bright area, so a smaller ep may suffice.
I believe there is a 35 Pan for sale in the classifieds ATM, I have used one and they are really nice. 68deg AFOV, pin sharp nearly to the edge and surprisingly good contrast for such a low power EP.
Cheers
Malcolm