Hey Nikita.

I just finished this step a few weeks ago. Here is how I went about it.
First asemble the center section with the booms pulled tight against the center section. Measure the front of the booms from center to center(I marked the center of the booms on a piece of paper to get a more accurate measurement.)
and the rear edges of the fins. You should be at least within 1/8"

Now you can determine if you need to use the balsa fillers. I'm 99% sure you will still need them, but it is no big deal. Since the bolt holes aren't on the center line of the stab, mark their location before you glue them on. Once you get them glued on you will still have to sand and carve them down to fit the curve of the fins.

If you use Evercoat 408 to fill the small gap that is left, cover the inside of the fin with wax paper and NOT masking tape.
My fillet stuck to the tape and not the wood. So I had to re do it.

Good luck
Jeremy Smith