The engine tuning data on page 05-1 of the workshop manual give a table of mechanical advance figures (these are on top of whatever advance you have set). However, I've found newer distributors tend to have a different curve with no mech advance before 1000rpm - I suspect this is to avoid hunting when the idle is set higher for pollution controlled vehicles which are set to idle at around 800-900rpm.
The table is:
450rpm - no advance
600 - 0-4
900 - 4-12
1200 12-16
2500 22-26
3500 30-34
4500 38-42
That is with the vacuum line off. I run with about 8-12 degrees on top of that which is maybe a bit aggressive. It does pink like that on 95 unleaded at 12+, so I tend to run 98 which seems to run nicer and faster. If you're using lead replacement, octane boost you may be able to get more or less.
I'd have a play with the gun and see what advance curve you have and plot it out on graph paper and compare with the table above. If it's miles out you may need to strip down and re-grease the weights in the dissy.
I found my NOS Lucas 45D was close but not identical to the curve above the difference being that it was more of a straight line. I ended up removing the lighter spring and stretching it a bit to get a more aggressive ramp up at lower revs.
What you can get away with and what works best will depend on how good or bad your compression is, how your carb is set up and what carb you have (among other things: vacuum advance, compression ratio, how much build up on piston crowns/head).
I'd start at about 4 degrees advanced and then keep advancing in 2 degree increments until you start to experience pinking - you'll hear this as a rattling from the engine when under medium load (say 4th gear at about 30-40mph pulling up a gentle hill with your foot down). When you start to hear it back the timing off a degree or two.
The curves in the workshop book are assuming of course that you're running on 3* petrol...
I'm starting to collect together parts to make a data logging system to monitor mixture, rpms, vacuum, acceleration, speed, engine knock and distance with a smart phone and a microcontroller.
Tuning engines is addictive, especially ones like the Landy petrol which has lots of scope for improvement from stock. If I get time between rebuilding my 109, moving house, working and bringing up a baby I'm going to have a go at designing and building my own electronic ignition system just for the jollies!
Good luck and let us know your experiences, it will be interesting to see how yours pans out.
PS: I wouldn't bother with fancy air filters, I ran a racing cone filter for a while because my new carb didn't fit the pipe work. I went back to the oil-bath. Less hassle, a lot quieter and I couldn't measure any difference in time on a 1/4mile standing start.