I have set 150 m (500 ft) safety altitude, and I monitor required L/D versus actual L/D.
I exclusively fly in the Alps with a 47:1 ship. A required L/D of 20-25 is safe in almost all conditions. Anything between 30-40 might work, but you need a plan B. Did a 50:1 over 90 km this summer to return to Fayence after thermals died (same flight as in Philipp's video Fayence-Furka-Fayence), but only in almost still air with 10 km/h of tailwind.

Bert
Ventus cM "TW"