I would say set the QNH at transition. In that way if the controller asks "Report passing F200" you would give a REAL report ...Esp if the QNH differs from 1013 markedly.
ops wise I set QNH on the standy alt once assigned an ALT vs FL and then set all alts to QNH at transition ( or a few 1000 above if you get busy on MBZ/Area/HF/.....
)
MAx