My nether portal is on a friends private realm and it keeps acting up... the whole realm has been having strange activity.. but as for my portal, it's coordinates in the overworld are 2042, y, 4201... and the coords in the nether should be 18, y, 108 (a good estimated guess). Sometimes (3rd time now) when I enter the nether portal, it puts me in a whole new location.. the new nether coords become the coords from the over world.. and when I go back through, it puts me +30,000 blocks out!.. very annoying.