Setting sag

Almost every bike will come with a specific desired sag, for the rear shock in particular. Suspensions are designed around the neutral point being in a particular spot.
Take my DW-Link equipped Ibis. DW-Links are on the opposite end of the scale from plush, whatever term you'd like to use for that (firm, harsh - I don't know). They are also relying on a millimetre perfect neutral position to realise their best behaviour.

I once did a skills session with a woman who had a DW-Link equipped bike. She had so much air the shock that she had approximately zero millimetres of sag. None. I explained to her why she should change that - effectively she was on an expensive and heavy hardtail. She wouldn't buy any of what I was telling her. I don't know why she wanted a session with me if she was so closed-minded? Someone she trusted told her something she interpreted this erroneous way. I tried.

My rear shock has 45mm of travel. The sag point is at 1/4 of travel - or 11mm of shock motion. It is easy enough to bounce around on the bike seat, reset the o-ring on the shock shaft, gently get off the bike and measure to see if it is 11mm or not. If it was 13mm I would add 20% to the air pressure in there. If it was 9mm I'd take out 20%.
Remember that the optimum sag in a rear shock is dependent on the bike it is installed in. Some call for 20% sag while others work best with as much as 35% sag. For the Ibis this is 25%.

One minor thing to think about is that the brand new shock is full of brand new surface and brand new seals. These things are a bit stiff with each other. The shock won't be as free at hour zero as it will be at hour ten. Set it close and then check it again after a few rides. Then check it again after several more. It should be good at that point for the foreseeable future.

Forks are both more and less predictable than shocks. More predictable because they behave the same way on every frame. Close enough that Fox and RockShox put pressure guides on the fork as a reference. Less predictable in that I have never gotten along well with the printed air pressure for my weight. It is usually too much for me.
Whether my XC fork or my DH fork, or something in between, the recommended fork always proves too firm. My Fox 32 never achieved full travel despite sagging to the desired 20% (20mm in this case) position when I'm on the bike. My Fox 36 TALAS also felt too firm with the recommended air pressure inside. The 24 on the Ibis is inflated to the recommended pressure and I have never come close to using full travel yet it sags almost 30% - suggesting too much ramp in the air spring.
I always run my Fox forks with lighter-than-Fox recommended fork oil and zero compression damping. That just feels best to me. I have tried running extra low speed compression for steep and fast riding, but it seems more likely to make my arms sore than it is to return better handling through less sag. Adding extra high speed compression in the Fox 36 doesn't add anything good, it simply makes my hands hurt from the extra jarring.

Definitely experiment. Keep a notebook (or a digital one on your phone) so you can record air pressure versus experience. Most rides you should get close to, if not all the way to, using all of your travel front and rear. If the "ramp" of your suspension is not sufficient for your use of the bike then you could bottom out harshly at speed when things are set to standard settings. In this case add some air until the times you do bottom out it happens gently. Harsh bottoms can damage the frame and the shock/fork.