Overflow box finger depth

Not really sure where to put this
I have a 55 gallon tank
A 10 gallon sump who's only purpose is to hide my pumps and heaters
I have 2 pumps both rated at 700 gph
1 plan on having 1 supply my algae scrubber and 1 to return water to the display tank
I drilled 2. 1 1/4 holes 5" from the rim of the tank and built an interior over flow the dimensions are 36" x6 1/2"x1 1/4"
On the exterior it flows into another box which is 17" x7" x3 1/4"
Both boxes are made from 1/4" acrylic
All plumbing is 1" PVC
So long story short my question is
How long should I make my fingers?
I was thinking 1/8" fingers with 1/8" spacing between them 1 1/2" long the 36" length of the interior box
Thanks for the thoughts and help
Randy

Most of the ones I've seen have 1/4 or 3/8" open spaces, but other folks will know much more about this.