For me the fit sequence comes down to something like this…at least for a general use trail / XC MTB…
1. Pedal-2-butt fit be setup so that extended seated pedaling has proper leg extension and kinematics (not too knee-forward or knee-back from the pedal spindle). Dialing this in takes a combined tuning of saddle height, saddle setback, and crank length *based upon* the rider’s thigh, calf, and foot proportions. Like you have done, this can be tuned with different saddle fore-aft adjustments and various degrees of set-back on the seat-post. Seat post extension is a factor because typically as a post is raised or lowered the saddle will move for-aft depending upon how slack the true post angle is. For me this setup results in a position that can handle sustained efforts both on flats and climbing without excessively stressing any joint and while spreading the effort amongst all muscle groups. The hips and knees should be moving through smooth and full ranges of motion, again for me this has proven to provide the best long-ride endurance. Your butt should not be too far back, too close to a vertical dropped through the rear axle, if it is then this leads to having not enough weight on the front wheel. This reduces front traction, particularly when climbing, and thus control.
2. Butt-2-handlebar fit likewise is tuned starting with a seated pedaling position. The target is a rider-specific setup to be sure but I’d be looking for a body position that has the upper body at a moderate forward lean, the neck/head at a comfortable angle for extended periods with a comfortable forward and some upward view, the arms with a bit of bend at the elbow and a fairly neutral wrist angle, and the hands far enough apart to not restrict breathing in any way but not so far that there is excessive weight on the hands. I feel the rider should be able to maintain the same upper body position and ride no handed, at least briefly, without becoming unstable. This defends against surprises that cause you to pull hands-off now and then. The primary physiological factors that drive this are rider torso length, upper- and lower-arm length, and shoulder width. The adjustments on this come from the stem length and angle; handlebar width, sweep, and rise; and headset spacers or lack thereof. A sort of rough baseline to start with is having the handlebar grips *about* the same height as the saddle … there’s no hard line here though because the of rider proportions and rider preferences the individual may want more forward lean or upright body. Likewise if the rider has hip flexibility issues they may need to start with a more upright position (to reduce range of motion requirements) and possibly progress towards the more forward position. Under no situation should the rider end up with a vertical spine as that would transmit too much unbuffered shock from the bike to the spine and neck, even a slight angle here can be a significant safety guard and may prevent repetitive stress and other injuries.
All this of course starts with the bike configured for a neutral trail riding experience. With bikes having varying levels of adjustments to wheelbase length, head angle, suspension travel, etc., all those factors can change the rider balance point of the bike.
Then, I like to make sure the rider can move around properly off the saddle. Stand and pedal without unbalancing either wheel overmuch. Crouch and pedal. Get their butt back beyond the saddle and then get back on top. Pedal from a scrunched-forward-on-the-saddle position, et. al. Specific fit adjustments may change to accommodate these activities.
I feel that a rider should be able to handle anything in their skill envelope handily from a crouched position on the bike, butt not on the saddle but a couple inches off, allowing them to move their body fore/back/left/right to handle technical situations and allow the bike to flow with the terrain.
Once that neutral position is figured out a hard-tail or FS bike of course needs to have the suspension setup properly baselined…e.g.: set fork and shock sag. Each bike has a designed target that was in mind when it was, er, designed, and that should probably be followed at least initially. All riders have their own preferences and will likely tune their bike suspension a bit over time as they get to know the bike, and fit it more specifically to their needs.
Likewise, each rider’s style and choice of trails and activities may affect the best setup for the bike…someone riding predominantly downhill (e.g. on a DH bike) will likely have a much shorter stem and somewhat or dramatically wider handlebar.
There’s a big push in the industry for trail riders to have short stems and wide bars too, not just DH bikes. I feel like the Enduro racing style and promotion is what is driving this, but there’s no contesting that using wider bars gives you more steering control – it’s simply increasing mechanical leverage in the rider’s favor. Shorter stems are the compensating factor here, taking what would have pushed the riders hands too far away and pulling them in a bit with the relatively shorter stem.
I’m actually trying this out on my own personal bike (swapped 710mm wide bars with 90mm stem for 780mm bars and 55mm stem) but don’t have enough miles yet to say Yay or Nay yet. I’m accustomed to that bar proportion already though, it’s the same as on my DH bike.