make it larger by a half inch or so (see the math in the above posts) and then 'square up', which in this case means to trim to the size you want with the 60 deg angle... you must start out with more than you need or it will never work... just like the HST...we cut larger on purpose so there is something that allows us to trim up WITH the seam rather than the outside edge...

Jan--you nailed it--none of my rulers--even the new one--have the 60 line at the corner! That is why this is a struggle for me! I have about five square rulers or various sizes--it wouldn't have taken much to put one on! Sigh. I've made many HST with this trim method with perfect results... but at least now I know WHY I'm stuggling. Thanks you!

ghostrider--thank you for that link! I feel better I'm not the only one. lol

Joanne's has a 6.5" one on-line for $5, but I'll check the store first. It takes them a long time to get a package to its destination and the shipoping is more than the ruler.

Karla...a ruler isn't going to make it work with both the angles and side lengths you have given, finished or unfinished. It's just plain impossible. Save your money. The math doesn't work...just ask Pythagoras. See for yourself with this interactive triangle.
http://www.mathopenref.com/triangle306090.html
Or draw your 2.5 x 4.5 rectangle on graph paper, add the diagonal and use a protractor to measure the angles. None of them will be 60º...it's 90º, a bit more than 61º and a bit less than 29º. Those bits make a difference. Forget the angle and go with the side measurements so the HRT fits with the rest of the block. The instructions are just plain wrong.

I'm just going go draw a line on that will work. ;-)

I wonder if the Tri-Rec ruler would be helpful for this? I bought the ruler but
haven't used it yet. You can make Half Rectangle Triangles with it.

Hey EasyPeezy--I asked if the Tri-Rec would help when I posted the block--but no one answered. I have them, but haven't used them yet... I wonder too and will take a closer look. Thanks.

Let us know how the Tri-rec worked.