Help design weir overflow

[rlugo29]

I need help determining the height and length I need to cut out for my weir overflow. Tank is a 20L and I'll probably go with a 10-20g sump. I plan on buying the SCA-301 skimmer that runs the Atman PH1100 Skimmer Pump with Needle Impeller which normally does 365gph, but I'm told does half that when used on the skimmer. So, 183gph max. I like the idea of having the return pump match the skimmer so I need to design my overflow to handle 200gph, maybe 300gph max. I need to know the size of the cuttout I need to make to handle that gph. please keep in mind that I'll be cutting the tanks plastic bracing off and euro bracing the tank,

 

Also, what Dremel bit do I use?