It seems to me that regardless of the particular model variant, all Dell'Orto's require roughly the same settings from the better types {VHSC's) down to the PH's so popular with many Lambretta owners,
My experience has found time after time that slides of 40 to 45 are what are required, with a definite tendency to 45, even on full race engines.
Unfortunately, even though Dell'Orto's are inherently tuneable by virtue of the massive range of component parts available, slides are not cheap. I would suggest borrowing a 45 slide if possible prior to buying outright. If a 45 cleans up the flat spot, a 40 can be modified with care to become a 45. It's just a matter of removing a small amount of material from one edge, a practice fairly common years ago with the likes of Taffspeed etc, I don't know the amount off hand, but it can be readily calculated by comparing a 40 with a 50 & halving the difference for a 45. On one occasion, even a DIY 42.5 was made to suit a VHSC,
Before anybody else questions the atomizer size, 264 should be fine. A bigger atomizer will result in loss of fine fuel control. If in doubt, run the engine on the stand with no main jet fitted. If it does run anything like when a main jet is fitted, there is a problem - if it runs like a crock of sh1te, then all is well & pop the main jet back in,
The only other thing worthy of mention is the float valve. Theoretically, 200 is fine but IMHO a bit near the edge in terms of safety margin. Whilst I do not agree with 'the bigger the batter' as so many seem to advocate, if buying some other parts, you might want to add a 250/300 to your shopping list,
I hope my suggestions help in some way