Not so long ago, I used Agfa Geavert, probably XT125. It was about 12-14 y.o. We didn't do any sensytometrical tests. Just assumed, that it lost about 4-5 f/stops so we exposed it as a ISO 3.
We could only shoot from 11AM till 1PM, only in sunlight (winter, snowy exteriors). And indeed we managed. On telecine the colorist couldn't believe his eyes. There was no green layer, just a bit of red and blue. The density was different from left side of the stock to right. It was looking more less like a cross process with a bit of LSD look :-)!
Try to find a project that You can experiment and just do it. That's my opinion. And overexpose! As much as You can.