Here's a ruler I adapted for a current project where I had a lot of HST to trim down. The tape is 1/4" seam tape and shows my desired final size. The diagonal line lines up on the HST. On the bottom of the ruler, I cut a clear finger sized bandage in half through the middle of the pad part and stuck those on either side of the seam allowance to allow for the teeter-totter effect of the seam.
So, yes, I have to trim but it is fast and easy and super accurate!