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Chris, you are forgetting, she's a woman, she capable of multi-tasking.
But seriously, whatever, if someone else wants to map thats fine.

Lol.
Can be done with a pause at the end of every corridor run if you prefer. It probably more time efficient to do mapping on the fly though. 1 action to put away.

For clarification Jim ideally would like to be at the outside range of the dim light range that would be provided by the party when scouting but is not adverse to going beyond that if the party wishes to hang back.

In my prior post I outlined his next couple of actions so there could be some progress without me having to check the forum every half hour. Ideally he would have the party follow 30 feet back of him when going back to implement the coin toss experiment, my mistake for not mentioning that.

To Nix before heading back down the hall "That could come in handy Nix. Thank you for sharing that, knowledge shared is never wasted."

Sue will walk up to the second corner as requested by the scout.

Asking again to see if the group will cover the light source.

Letting the crossbow fall for now Alecia closes her hand on the light and resumes taking notes on the corridors.

Leaving the rest of the party behind the last corner Jim moves as quietly as he can to within 5' of the top of the stairs. While Alecia updates the map, Sue stares at the adjacent wall & Everwood concentrates on the ceiling. Both are dressed stone. Alecia covers the Light as requested which is Jim’s cue. He lobs a coin into the room and then sidesteps to the left corridor wall. The coin makes a coin-hitting-stone noise and rolls out of sight behind one of the pillars near the middle. He hears an incredibly faint noise, perhaps best described as scuttling. Maybe a rodent or small mammal..

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If we’re happy with the 40-45’ lead by Jim and then the rest following then we’re good. I will treat corridors as double move if required but single move using an action to map at the end of each corridor. This allows me to measure time elapsed whilst not having to bother the party with it.

"Sue, take the lead. Move ahead closer to Jim. I will follow 5' behind".

With the light covered. "Anyone notice anything different in their sight?"

"Let's see what Jim found out." He will close half the distance to Jim.

Sue will ready sword and shield and head to the stairs unless Jim stops him. He will take a good look at the stairs before heading down.

Perception(Wis)(2): [1d20+2] = 19+2 = 21

Jim sees Sue approaching and moves 5 feet back to meet him. As Sue comes abreast of him Jim reaches out to place a hand on his shoulder to halt him momentarily. In the slightest whisper Jim says "small motions in the room, maybe rodents, maybe something more. Be prepared. I follow you in." Easing his mace free he falls in behind Sue while gesturing the group forward.

seeing the gesture in the dark ahead (because the light is shieded) nix makes a silent sign to Alecia that they're on the move again. He repeats his magical-sight incantation extremely quietly and walks up just behind where Sue and Jim have stopped (not leavong Alecia behind)

Nix realizes that that for Alecia it's completely dark down here and she can't see his silent sign but a whisper and aided movement (speed halved) can achieve the same without revealing the light.
Eventually the whole group reach the top of the stairsa, Sue on point, then Nic & Jim with Everwood and Alecia 5' behind.
To those with darkvision the widening room below is filled with lots of stone pillars. Archways can be seen in its northern, western and southern walls. All seems silent below.
To Nix's witch-sight the corridor and stairs have the same blue abjuration aura that he'd seen before but the pillared room does not - not a grain of magic to be seen.

No magic near the pillars. he whispers to those in front

Jim gives a nod of thanks to Nix and turns back to Sue. In a bare whisper he says "traps I know not. We move on you."

Evenwood says to the team, "Let's move in as a group."

Peering down the stairs Sue sees a widening room filled with lots of stone pillars. Archways can be seen in its northern, western and southern walls. There's something tiny and motionless on the floor among the stone pillars but it's too far away to see what.

Without uncovering the Light again Alecia is going o have a hard time getting down the stairs and certainly won't be able to map.