EasyPeezy

So far the best tool I found for making "perfect" flying geese is the
Bloc Loc ruler. You cut bigger, sew then trim with the BL ruler.
If your squares are not accurate to start with maybe your ruler is slipping
while you cut. Put some Nexcare tape under the ruler to prevent that.
Check your table. Put a level on it and check if it's perfectly level.
A lot of tables have a bow in the middle. Found that the hard way.
I make my cuts closer to the edge of the table for that reason.
I also starch all my fabric before cutting to avoid stretching.
It's whatever works for me.

Edit: I have the stripology ruler. Worked well at first but with time I guess
my table had a bigger bow and it became more of a pain so I gave up.