A good formula to calculate the minimum acceptable diameter of vent comes directly from R.Small, and according to him is 'strongly empiric':

Min_Diam_of_Vent = 0.56 * D(Speaker) *SQRT(Xmax)/SQRT(SQRT(Fb))

Measures in millimetres, Fb = tuning frequency in Hz

Now the lenght can be calculated with:

L = [c^2/(16*PI) * D^2/(Vb*Fb^2)]- (0.85*D)

Where :

D = the vent diameter previously defined

c = airspeed (normally 344 m/s)

PI = 3.1416...

Fb = tuning frequency in Hz

Vb = box volume in litres

In case the duct is too long/big, a good trick is to shape it in variable diameter, typically in sandglass shape (2 cone frustums joined by a constant diam.cylinder).

A software to calculate this vent is here, for those who can check an italian dictionary (click on ACS icon).

http://www.audiocarstereo.it/software page.htm
About the room gain question, it is very difficult to answer.

There are so many variables, like speaker emission (closed, vented, bandpass etc.), relative position to walls and floor, room dimensions and reflection rate etc.

I think it is impossible to make a 'reasonable' estimation with manual calculations, the sole CAD I know who can give some help is 'Audio for Windows', that is handicapped for being available only in Italian.

(the CAD is able to simulate speaker alignement, box dimensions, speaker position in the box, active or passive filters, room parameters etc. in the same project)

sandro