It might be easier for you to mark your center cutting line with a ruler with a 45 degree angle line. Place that marking on the edge of your fabric and then use the cutting edge of your ruler to mark your fabric. If you have a 1/4" foot that should be fine for sewing your seams on either side of the drawn line.
Generally, I need to trim up my HST's so I cut the squares at 1" vs the 7/8" measurement. After they are sewn and pressed, I then place my 45 degree line on the seam line and trim to the measurement I need. I can't sew an accurate HST any other way to save my soul. What I do is more work and tedious to be sure but I get accurate HST's to continue my project.