Have you tried using a June Tailor shape cut? This guides your rotary cutter so you can cut straight strips. Once you have strips, you can align the strips under the cutter and cut squares.
You could also consider investing in an Accuquilt Go! This is a die-cutter, so no rotary cutters involved, and it makes very accurate cuts. I prefer it especially for triangles because it cuts off the little excess corners, making matching very easy. Prices have come down a lot on the Go's, so you can get started with one for about $200 nowadays.