Just a couple of pointers; the formula used is the one that is generally used for 'displacement hulls' is not used for the type of catamaran your design depicts. The other minor point is the term 'small cat', present terminology favours the classification "Beach Cat"
Probably the best example and nearest to your design would be the Hobie 16 :-
The Hobie 16 is a 16 foot / 4.9 m catamaran with two sailors and has been a popular racing catamaran for 40 years. Hobie Cats were born in a surfboard shop in Southern California in 1969 and are named after the designer, Hobie Alter. Numerous Hobie catamarans have been designed over the years with the most popular racing boats ranging from 14 feet to 20 feet in length. The boats are easy to launch from sandy beaches, physically rewarding to sail and popular all over the world.
If you go to this link : http://www.hca-na.org/
then on the top right hand side click
on 'resources' then on 'Technical information' where you will get :-
Line Guide (all boats)
Wire Guide (all boats)
Hobie 16 assembly manual
Hobie 14 tuning guide
Hobie 16 tuning guide
Hobie Tiger tuning guide
Hobie FX one tuning guide
The above information may be useful in determining the placement of control hardware and provide a guide to correct line and wire sizes on your beach cat.
By the way, the H16 beach cat has recorded over 22 kts on several occasions (there may be some official record with a much higher speed)
Here is a U tube of a Hobie sailing at speed