I had a question about how to measure square. First and foremost, find a ruler brand you love! It is true different ruler brands measure slightly different. The width of the line is often the culprit. I have seen some rulers with a 1/8 inch line. Think about this... your pattern calls for 3-7/8" squares. Add or take away 1/8" line, it could distort your entire quilt or cause issues down the road with piecing. The thinner the lines, the better off you are. If you love your ruler and it has thick lines, that's okay too. Spend some time learning where to put your line for consistent measuring.
Recently I needed to replace my old rulers. Their lines were rubbing off and the edges had nicks, I wasn't getting accurate cuts anymore. When I went to the store, I noticed the lines had tripled in width. Granted, it had been about 12 years since I got mine. I went ahead and purchased it thinking I would get used to it or perhaps love it. Long story short, I now use Creative Grids rulers. They are not cheap! I would rather invest money in a ruler so I won't have wonky measured fabric.
Bottom line, regardless of rulers, remember these things:
1) It might seem like a "duh" step, but know what size you are supposed to cut.
2) When you lay your ruler on your fabric, make sure the lines are on the correct line. Sometimes I will use the Post- It Note arrows just to be certain I am cutting on the correct line.
3) Make sure the line is ON the fabric. The edge of the fabric should be aligned to the left of the line.
4) Use a SHARP rotary blade. I know they are not cheap and often we use them long past their expiration point.
One last thing, just like more people get injured with dull knives, the same could be said about rotary cutters. If you have to put much effort into cutting, change the blade.