Auto-x spoiler help request

I race auto-x in the XP class in a 07 Honda fit, last year after a lot of trouble trying to tame the tail happy nature of the car, I ended up fabing a rear spoiler to try if it worked, turn out it did.

what I did was a very quick and ruff experiment with what I had and wasn't up to the edge of the rules.  I"m now looking at re-doing it and I wonder how much I should do.  Here's what I have now, it's about roof width:

I'm wondering if I should go wider than the roof and down the sides (see red line) and if it's worth it. Also, should I give it more (or less??) angle?

here's the rules about that so you don't have to look for them:

P. A spoiler may be added to the rear of the car provided it complies with either of the following:
1. It is a production rear spoiler which is standard or optional equip- ment of a US model of the vehicle or an exact replica in an alternate material.
2. It is a non-production rear spoiler which is mounted to the rear por- tion of the rear hatch, deck, or trunk lid. The spoiler may extend no more than 10.0” (25.4 cm) from the original bodywork in any direc- tion. Alternatively in a hatchback, the spoiler may be mounted to the rear hatch lid at or near the top of the hatch in such a configuration the spoiler may extend not more than 71⁄2 inches (7.50”, 19.1 cm) from the original bodywork in any direction. The spoiler may be no wider that the bodywork. The use of endplates is prohibited. Spoiler endplates are defined as any vertical (or semi-vertical) surfaces at- tached in front of the spoiler which have the result of capturing and redistributing air (downforce) along all or any portion of the spoiler. The angle of attack is free. The spoiler may not function as a wing.
3. All OE rear wings and rear spoilers may be removed.
4. Vehicles equipped with an OE rear wing may add a rear spoiler only if the OE wing and wing attachments are first removed.

thanks for any inputs!

XP! What have you done to that fit! Sounds like it must be awesome.

What is your goal with the new splitter? More downforce, more adjustability, something else?

In reply to Robbie (Forum Supporter) :

Fully gut interior including dash and heater box, turbo'd (166whp/166 wtq)...  

plan for the Spoiler (not splitter ;) ) is to get as much down force on the rear as possible, like I said, car is a tail happy bitch...

this fit sounds awesome, i vote for start a build thread, Id like to see more of it.  

for autox spoilers youll want as large as the rules allow, with the most angle to m ake the most downforce.  make it as big as you possibly can, and then tune it to your likeing by raising or lowering the angle.  At least that would be how I would go about it.  

 If you look at this one, I went all the way out as far as I could to get the most out of the spoiler.  This was an ESP car so I think you have a little more available in XP.  
 

As said, go big. To make full use, you need as much power to run it. If you can make the sides removeable on it, or adjustable do.  Then you can test and see if it's slowing or helpin you.