[I.C.D (Ian)]

Hi Can anyone help me .I am trying to install a OAG to my Meade Lxd 75 8" SN .I have a JMI EVN focuser ,Limuicon OAG and a Atik 420 c camera and using a ZWO asi 120 mm mini as aguid camera ..From the prism to the atik i have 33mm +/- 2 from the prism to the zwo i have 43 +/-2.Do I need to put a spacer in where.
Ian