QB, Don't get frustrated! I have found that using a finer but still 50 wt thread is the key for me..My first go round I had the same problem as you.. I still use C&C thread but find it's fatter than let's say other brands. I won't use it when I need to be more precise. Another factor could be the weight of the fabric. Smoother fabric will lay flatter than fabric that has a courser feel to it. I sew 2 pieces together, fingernail press then measure to see if its 3.5 inches I do this to every section.. It takes me longer but at least I know at that point whether or not to rip the seam out and do it over after making adjustments either with the machine or my handling of the fabric as it sews..I can't sew a straight seam without a guide on the presserfoot.. my seams tend to be short on one side because of the way I hold the fabric then I have to go over the seam to make it straight.
Whatever you want to send is fine by me.. there is more than 2 weeks to get done and sent to me.. I still have a ton to make..I haven't been feeling well the last 2 days with a very sore throat and aching body.. I was supposed to be sewing today and I'm still in my PJ's.