Since I just took my Savage from the workshop, and since I've yet to meet a mechanic who knows how to set the decompressor, I decided to recheck it, and I think I found a fast and easy way, without removing the gas tank or engine covers:

-Remove the timing port cover
-Crank the engine until the grooves indicate TDC. I can say whether I'm on the exhaust or compression stroke by how hard it is to turn the engine (on compression TDC I can feel the compressed air pushing back, on the exhaust TDC there's less resistance, because the valves are open).
-Push the decompressor magnet by hand and check how far the shaft extends - should be 3-5mm.
-Adjust the nuts on the cable (it can actually be done without removing the engine cover) until it's OK.
-Give the engine a full rotation and recheck on the other TDC. On the exhaust stroke the valves are lower, so there will be more play on the electromagnet. So whichever position gives me less movement of the electromagnet, it's the correct one (compression TDC).

So... is this a good way to check it, or am I missing something and I can damage something checking it like this?

Until what I'm adjusting (the electromagnet travel) is OK

Mostly I wondered if finding a compression stroke by feel is correct, and if I can do the adjustment without removing the spark plug and valve covers. Clymer suggests removing them, and guides on this forum mention removing at least the gas tank, so I wanted to be sure.

The slack was set to 0 (I could barely move the electromagnet) - I had a feeling it would be that way and wanted to check it quickly