So, the way metal yields is defined by a graph for each metal. When you see "torque to X and then Y degrees", it's saying torque to the high end of the elastic region, then turn till you're in the plastic region.
Anything in the blue region is "elastic" and causes no permanent deformation. Anything in the green region results in permanent deformation of the fastener. Interestingly enough after being plasticaly deformed steel will follow it's original stress curve back down parallel to the original curve from the point on the plastic curve.
In theory if you can get a torqued TTY fastener exactly to where it was when you removed it on a stress curve you can reuse them almost indefinitely. The problem is actually doing that. Torque is only an analog of stress, and further, because you've elongated a portion of the fastener from how it was when you initially torqued it, the same torque applied to a used fastener creates a different stress level from a new fastener, which is greater. And no, there is no conversion factor to use here, if such a thing did exist it would be highly dependent on the exact material used in the fastener.
There is also some science behind deformation being an important part of not backing out, but stress/strain is what governs clamping load.
If you do reuse a TTY fastener, you're probably best off measuring the breakaway torque when removing it (with a digital torque wrench this is pretty easy and not too time consuming) and retorquing it to the same number. If you don't want to bother to do that, you can probably multiply the torque value specified in the manual by 1.15, for an additional 15% of torque to reach the same strain as originally applied by the "torque to X and then Y degrees" pattern.
For the most part it's really a question of "Is it worth the cost of replacing the fasteners to not have to do the job again." On my VW when I did the camshaft the bolts for the camshaft caps came out to $200 through the dealership after my aftermarket bolts got overtorqued by accident and I decided that yes, I probably should replace and not reuse the fuel injector rocker bolts as the aftermarket kit assumed you should. That was the borderline point for me, where I was strongly considering just reusing the over torqued bolts and hoping for the best. (Torque spec was 7ft-lbs and then turn 90 degrees 3 times in a pattern, the 7 ft-lbs was exceeded by a screw driver.)
Head bolts are what, $70? Totally worth it. Now, bottom end bolts OTOH are very hard, if not impossible to find, resulting in more of a case for reusing them. When I make a choice to reuse a supposedly TTY fastener I look at a few things. How critical is the part? Will I die if this fails? Is this fastener ON a rotating assembly where it's more likely to back out? Is there obvious evidence of elongation on any of the fasteners already? Are replacement fasteners easy enough to source? Could I just replace this with a non-tty fastener and still meet my general safety requirements.
