How much thread is enough?

[svadagio]

What is the minimum amount of thread to reach full strenght on a turnbuckle. I have 5/8 turnbuckles and have 1 1/2 inch thread each end. I seem to remember that 1 1/2 times the thread diameter is enough but I think that was bolts. That would be about 15/16 in my case. Anyway I am interested in what info you have to share.
Thanks
Bob