Time to remove the forks and replace fork oil.
I'm using 
Ban Chang Motorcycle Tire Shop & Service on the spanners.
The oil refill will be 
Ohlins SAE10 fork oil at 90mm from top of fork leg, as per the official Honda ADV350 Tech Manual.
Interestingly the YSS fork upgrade provides conflicting fork oil volumes on their YSS web presence and the YSS upgrade kit installation leaflet. Also YSS provide conflicting oil viscosity (5w and 10w) data. YSS HQ have failed to answer my query for clarification, sent by email, and didn't get very far with a phone call either.
So my position is that I will 
rely on the Honda data, at 90mm fill height. 
This fill height 
might give the Honda volume of 
403ml, but also the YSS upgrade spring 
might itself take up more physical spring volume, hence relying on the 
Honda 90mm refill height as the absolute.
I don't want to blow seals by over filling and not being able to compress oil, resulting in blown seals. That would be disastrous.
Update: fork service completed in 2hr 40 minutes, sometimes with two mechanics working together.
Here's the YSS conflicting fork oil volumes....ones states 340cc and the other 380ml. (There's no conversion difference between cc to ml).
Also note to conflicting YSS oil data for kit#004 web image states 10w and install leaflet states 5w (and Honda state 10w, which is what I will be using). The YSS fork oil supplied in the #004 upgrade kit was 5w.
Cost to replace fork oil, parts labour ฿2800.
I'll replace fork seals, dust seal and retaining spring at the next fork service.
Existing seal is showing no signs of being damaged/worn. So, I'll do a preventative replacement at the next fork service in another 20-25,000km