Always disconnect and plug the vac advance - that's what will be causing it to advance off the scale at idle (and high vacuum) giving you a false reading.

On a stock or mild motor, I generally start at the factory recommendation, and work up from there. 2 or 4 degrees more is usually OK - eg on the EFI 5L, GMH suggest 10°BTDC static advance, but I generally go to 14°BTDC, which gives it a little more response - but you have to have a keen ear to listen for pinging under load, and then back it off (or let the knock sensors do that - but the reaction time & resulting "pulling out" of timing bogs it down). Find a decent hill in your local area, short shift to 2nd to load it up, and accelerate up the hill - if it rattles, you have too much advance. Do this with a hot (operating temp) engine, as a cold engine won't ping as readily, due to the cooler combustion chamber.

Fuel choice will play a big part in how much advance you can add - running ULP91 in a carby V8 designed for super will put you behind the 8-ball to start with - you may need to pull timing advance out of it (ie down to a lower number) to prevent pinging. Likewise running 98 in something designed for 91 will give you a lot more headroom to advance it much further before pinging.

On a modified motor, it's best to seek a suggestion from the cam supplier, or if you've gone even further (more compression, larger valves etc) then getting the dizzy regraphed to suit is a good investment, and the workshop who performs this (you will need to give them all the specs) will suggest the best starting point for you.

Setting the idle mixture is when you normally use a vacuum gauge - aiming for the highest vac at idle. You can also do this without a gauge, as you can hear the revs pick up slightly at the mixture's "sweet spot" which corresponds with maximum vacuum at idle.

Thanks very much Mick & Dr Terry, that's what I was after.