"Dual" tuning a FX Cyclone

I would tune it for high power, with the transfer port on the biggest setting. Then just use the power selector to drop it down when you need it. That should be all you need to do. 

If you want to go sub 12fpe on High, you can flip the power wheel and it has a second set of 3 holes that are much smaller. I wasn't able to get near 12fpe on high with only a hammer spring adjustment. But with a reg, ideally you want a big transfer port. You are having to move more air at a lower pressure to get the same power

I regulated my Air Arms S500 (which has the same style power selector) and done it this way. When the power selector was turned down, it worked great. Shot count went up, and it kept great consistency. Even better than it did before the reg.

Here's another option:

The Cyclone uses a removable airtube. You could maybe buy an extra tube, and fit a reg in one, and leave the other unregged. Tune the hammer and put the port on Med or Low for the regged tube. Then swap on the unregged tube and flip to High for full power shooting or hunting. It may work out that you won't have to touch the hammer spring when swapping. Or not, I have no idea. But it should be close. 

I have an old Webley Spectre (Cyclone action) in .177, and I have considered trying it this way.