Hi Richard,
For me at least, the term "Gauge Flare" isn't helpful/may mislead. 00-SF has a track gauge of 16.2mm, which may subsequently incorporate gauge widening on curves as required (using well-proven tools such as 3-point track gauges) , depending on factors such as minimum radius and locos/stock used. Ultimately this gauge widening could result in a track gauge dimension of 16.5mm, for example. To me it's 00-SF (or 4-SF in Templot).
However, some (e.g. Gordon's excellent Eastwood Town layout) may choose to incorporate Gauge widening where not actually required, such as on plain trackwork of little or gentle radius. This may be due to several factors, such as a big layout is being constructed by one or two modellers and therefore the practicalities of building lots of trackwork isn't viable/isn't favoured. Therefore (as on Eastwood Town) the modeller favours the use of 16.5mm RTR trackwork where suitable.
For me at least it could be argued (pointlessly IMHO) that the layout isn't a "true" 00-SF layout. So what? It works for them and they're happy with it. For me I intend on my next layout to use 16.2mm 00-SF throught, using handbuilt trackwork only; it doesn't make it any better or worse, just more suitable/practical for my own personal circumstances and desires.
Brian