do you cut them 3 7/8 or are you adding the 7/8 to the 3 1/2? I usually add one inch to the 3 inch size and then trim for exact. some people have great luck doing the 7/8 inch bit though. but always add that to the "finished" size, not the unfinished size. and if you do trim always place the 45 degree line on the seam corner to corner, line up to size you want, like 3 1/2 and trim two sides. then turn the block, place ruler and trim the other two sides. try again, you seem to be doing fine.