Silverback SRS question (effective volume and cylinder:barrel ratio)

Little background:

I'm currently upgrading mine SRS with multiple parts. For now I have the following in possession:

• Rapax 2 and 3 joule
• Scorpion kit from stalken (piston)
• 18 inch fully fluted outer barrel
• The rest is still stock

And I'm planning to get the next parts:

• kraken ultimate edition hop up
• Maple leaf crazy jet barrel (6.04)

- silencer (DTSS .30 Silencer (Foam Filled - 14mm CCW))

The thing is that I don't know which length inner barrel is should get. I have read all about the inner barrel ratio's and all but there seems to be a difference in cylinder volume and effective cylinder volume. The volume of the SRS should be 41cc but I'm not sure whether i should use this metric as it is not the effective volume.
With the effective volume I mean the volume without the piston part residing in the cylinder.

As for the limits in my country/fields:
we are allowed to use bb's to 0.40 grams with a joule output of 2,3. Which means is that I would prefer a joule output around 2,25 joule.
I think someone mentioned that .for 0,40g 2,67 ratio should be used.
I prefer a lighter spring like the 2 joule so it doesn't feel like pulling the bolt is a complete work out.

So the question is:
What should be the inner barrel length for 'optimum usage''?

If some information is missing or is incomplete please ask! I would love to upgrade my SRS as soon as possible!