Scribe the cut line with a sharp scriber, cut a fraction long & then fettle to your lines with a 60 or 80 grit flap disk in the grinder.
A flapdisk will whip the excess material in seconds & will leave a nicely prepared surface.
EDIT
Common reasons for run off/out when cutting with a hacksaw
Not having the work piece level in the vice, I use a small level to ensure that it is.
Too much force, let the blade do the cutting
Standing in the wrong position when cutting, stand in a straight line to the angle that you want to cut.
not enough tension on the blade
Poor quality hacksaw or blade
Pesonally, I find it helfull to lower the heel of the saw when cutting ERW square tube. It helps to cut downwards whilst still making forward progress.
This way you can adjust both cutting directions as you go & before you run off line