We on the board can help- have no fear!!!! there seems to be 2 issues here 1) your seams should be 1/4" not 1/8" +/-. test the seam width by cutting and sewing 2- 2 1/2" strips togetyher, press. The resulting piece should be 4 1/2". If they are not then keep adjusting needle or moving fabric on the base until you get the perfect 1/4" seam. Be sure to put a mark on the place where you get the 1/4".
2) the 7/8 measurement refers to how much bigger you cut your square when making Half Square Triangles (HST). for exampole- to make a finshed 4" HST you cut 2 pieces of fabric 4 7/8", place right sides together draw a line on the diaginal, sew 1/4" on either side of this line and then CUT on the line. You cut on the line after you sew the squares together. I am not sure what ypou are referring to when you talk about triangles. Most patterns have you make HST using the aboue method. Lots of ladies make their Squares 1" bigger and then cut down to needed size. Why 7/8" because that is what the quilt police say!!!!!( and mathamatically that is correct).