Seam allowance really does make a difference on the fit of garment. Keep in mind that 19th century clothing was form fitting. If the pattern requires 1/2 inch seam and you're using 5/8 inch seam it will not fit. Case in point: at an annual sewing circle someone was sized perfectly for a corset. The owner of the pattern and another pattern maker pitched in to help. We did a test on cotton, made adjustments, then handed it over the wearer to cut the pattern on the material and sew the pieces together. When all done, it did not fit. It was too small... We were miffed and couldn't understand why. Then, eureka, the owner sewed using the wrong seam allowance. She had to disassemble the darn thing and start all over again. So... seam allowances really do make a difference. If you have trouble using the gage on your sewing machine, take a piece of masking tape and affix on the surface from end to end. If that doesn't work, mark the seam allowance on the material. Take your time. Practice. Eventually you'll master it.
-Yulie