If you are using the additional Avsim docs, check the performance2 doc and find this table.

The highlighted values are the optimal ones referring to your GW. The left side shows the corresponding PA (pressure altitude) to be used then.
So for example 265.000lbs GW leads to a FL370 cruise.

The same doc offers some fuel calculations too.

As for the taxi fuel consumption, you will end there with not real values as on most flight sim addons. FSX models the drag on the ground other than real and you therefor have to use some thrust where the real plane would idle most of the time.
Best way therefor is to watch your FF meters when doing a normal taxi.

Use the reset button on the eng station before and you get very accurate readings accurate which can be used on further flight planning actions.

Maybe this thread (http://www.captainsim.org/yabb2/YaBB.pl?num=1286907082) helps you a bit because the table above is just one part of the document which contains some calculation basis for your fuel consumption.

You are right, GW is the overall weight of the plane, including fuel, passengers and cargo.

I took another look at the docs and now I understand how the fuel planning works. 

It's just a matter of working the problem backwards.

However, what number should I pick for final reserve fuel?