Do you have a drill press by chance? If so, if the table is relatively level and rigid, open your drill vise to cradle the barrel or use a vee block if you have one. True your action to dead horizontal on the bottom flat where the guard screw goes in with a good quality level (prove your level by checking the bubble position and swapping the level 180 deg, the bubble should be in exactly the same spot in the sight glass). Take any true flat object, a machined and ground steel rule is ideal, maybe a very thick feeler gauge leaf, place it in a teeter totter position. Chuck a pointy tool, spotting drill or even a small center drill, lower the quill firmly onto the rule, adjusting the barrel and vee block/vise until the rule wants to be level under quill pressure...remove the rule, lower the quill firmly enough to dimple a witness mark, repeat for hole #2. The teeter totter is just as good as dials and DRO's if care is taken.
