Simply put this is a very low cost material cutter.
If you want accuracy every time then look at the Total Trimmers-Keencut and Dahle.
If you want a low cost alternative with a hand cutter here it is.
A great idea!
The Straight Edge safety ruler with a knife guide.
This is a 2 part kit- Ruler and Guide Knife.
Click on the image to get a better idea!