I concur with A&C. At one time my flying involved a significant amount of positioning a Slingsby T61, cruise sped 65kts. Into wind cruise at as low as kept me clear of low-flying rules (and with options if the engine stopped, although it never did inadvertently); downwind whatever height gave the best useful component.
Gliding in Scotland, anything up to 26,000ft until they started wanting transponders above FL195, gliding in South Africa, normally between 8,000 and 17,000 (on Mountain High EDS oxygen 100% of the time). 8,000 may seem a conservative lower limit if you don't know the ground is mostly around 5,500.