Do as above to straighten out the mast, also sighting up the mast helps too.
Then I "ALSO" recomend the loos guage. I did my rigging
by feel, came pretty close, but after borrowing a Loos guage, I found out how loose I still was in tightness!
If a tape is not availible to figure out the side to side straightening of the mast, the jib
or main halyard will also work.