I made elderflower in pet-nat style last year, and will probably do so again this year. The consensus seems to be to bottle at 1-1.5 brix (1.004-1.006 SG). That's a pretty tight window; it's probably too high the day before and too low the day after. And I'm sure you know this, but use champagne bottles...
Or, you could ferment it dry and add back sugar; 12g/L for lightly sparkling, 25g/L for champagne level of bubbles, though the latter is probably not a good idea unless you're controlling the process more carefully. And if you really want to go down the sparkling rabbit hole, you could do full-on traditional/champagne method (I'm hoping to bottle my
blanc de noir next month!)
Here are some useful pet-nat references:
The Science of Pét-Nat Wines | SevenFifty Daily
How we make Pet-Nat: frequently asked questions (and some math)
Technical Information about Pét-Nats (Pétillant Naturels, or Sparkling Wines Produced by Méthode Ancestrale)
Making it / 2014 – Pét-Nat.com