Yes - it's finished size of desired square (2 1/2 in this case) x2 (=5) + 1" to get a total of 6"
So if you want your finished unit to be 4" you would 4x2+1= 9"
There is very little to trim down but I have always gotten exactly the size I needed by doing this. It is so much easier.
I will say for larger pieces pin it to keep the fabric from shifting
I found that one out the hard way.