Grandma of 6

I'm working on a pattern that requires HSTs. It's not my thing! I'm not ending up with the 5" square I should be which probably means that my 1/4" seams are not always accurate.

I remember watching a video one time where someone showed a tool that you laid from diagonal to diagonal. It had a slit down the center where you could draw the center line and then you could draw lines on either side of the ruler for your stitching lines.

Does anyone know the name of the tool and where I could find it?

Thanks so much for helping.

Diane