a straight line with a long arm is only as straight as you can draw a straight line- unless you use a ruler-
there are special rulers for long arms= and (table/platforms) that fit the machine making the ruler work easier- if you want to do lines straighter than you can (draw) you would need to invest in some (attachments)
don't use a regular ruler you use with your rotory cutter though!!! they are too thin- the foot jumps right on them and breaks needles- causes damage- you need thicker rulers - who ever manufactures your long arm more than likely offers rulers, and other attachments to help with different techniques