I had trouble with bowing too until I got the new ruler with the built in strip cutter.
It works the same way the circle ruler works, slits cut into a ruler every half inch the whole way across the ruler. When I lay the ruler on the fabric after lining up and ironing, it covers about 18 inches of the fabric. I got mine at Walmart but I have seen them at Joanns and a couple of other sites and I paid 13 dollars.
Hope it helps, it helped me.