Welcome Bob. There is a very full and diverse font of knowledge here, so feel free to ask away.
when i'm re-making new rigging i'm aiming to have the 'finished' length with the turnbuckles at 40/50%. finished being the tuned tensioned length (usually docktuned), so when the rig is at sailing tension there is room for further tweeking if required. [this is for remaking existing rigging]. I'd use a toggled turnbuckle [or a toggle at the chainplate] n almost all situations, just to give good articulation. oh and right handed threads on top.
Good luck
