one thing about using metal is that it can hurt your rotary blade. if you use it to draw a line, it can slip and slide.
the other thing is that if it is narrow in width and thin in mm's it can warp, so be careful. maybe you should check that against a straight edge first.
i just treated myself to a 36" ruler for rotary cutting and i haven't used it yet, but i just got tired of looking for makeshift fixes. it will pay for itself in headaches.
i got it at joann's 40% coupon 'sale'.
there are no easy answers. just challenges.