A10 carb settings recommendation

[ghl]

I have a 1963 (built late 61, but must have been in store somewhere) A10 GF that someone rebuilt for me 3 years ago. All was going fine and it was run-in for around 1000 miles. It seized on me coming home, cruising at a steady 60mph - all was running fine up till that point (although it didn't tick over that well).
The guy/company that rebuilt it 3 years ago was used to working on old British bikes of all types and they have a 'collection' of all sorts of things theirselves - they had removed the Auto advance/retard unit and fitted a manual lever off the mag as said it was easier to get them running smoothly that way with modern fuel.

This time thought it was time I got my hands mucky again, so have had the barrels re-honed (lucky that they weren't badly scoured) and have fitted new pistons in case they were stressed etc.
I have checked that the left hand con rod has the hole facing into the flywheel etc - it was the left hand pot that seized. I had suspected the mixture to be too weak but having talked to a few people they thought it was a bit weak to not too bad.

So my question is, does anyone have suggestions of what to set the carb to, so that it runs OK on modern fuel - I have been running on super unleaded with Castrol Valvemaster (or whatever brand it goes by now).
It is currently fitted with an Amal 389 with a 3 1/2 cut away, a 160 needle jet and 250 main jet. The needle position is in the middle (3) with 5 notches available. From looking around this appears to be the standard setup from when it was new.

I thought I'd heard that when running on modern fuel you quite often have to run the mixture a bit richer - but any suggestions welcome