If I was totally organized I would have here in my office a chart or 3 ring binder with specs and could rattle that off for you, but I am not. In general you can find on the internet several charts for known graded bolts (IE: Grade 8 etc) and metal grades (materials) and there should be charts showing a correlation between stretch and torque. But its totally variable based on a number of factors you have to consider.
ARP has a proprietary mix of metals they use and you should follow their specific ratings, A unknown bolt from a vendor can be any number of materials. But the basics are,, a rod bolt is made to stretch under load. After a service life is exceeded, it wont stretch to spec. A well made bolt will stretch multiple times but eventually if it does not, then time to toss it. So that is WHY that is the proper method of checking.
That being said, on a tangent, some manufacturers specifically manufacture one time use only stretch bolts. I doubt BSA ever did this or many other period Brit bike manufacturers but its something to be aware of. Many modern auto manufacturers use these now and have for some time. Ford seems to love using them and can be expensive to purchase replacement head bolts. There is discussion in a number of forums about "WAISTED" Bolts and ARP makes some, and for some applications its a great asset to have. Some of the Norton people seemingly just discovered this application and are using it for their troubled and leak prone top ends which I find amusing as its been around a long time.
While many deride HD,, they did an amazing job with their all alloy engines, and I own multiple EVO sporties and Buells and their engineering staff performed miracles in getting the old iron head design into an all alloy engine that is so durable, leak and nearly maint free with few issues. If you study the evolution of the BSA Goldstar engine and the challenges they had, what HD did with their engines is nothing short of amazing. (Crdit where due) But to get an all alloy engine to seal, and stay sealed with all the expansion/contraction that an all alloy engine experiences is a feat in itself.