Mark Four: The Sallowmere Ritual
Nudge. The sun stood over the fire that never went out: that is where you start. North is up the plan. If you do not know how Holmes stepped the Musgrave Ritual, look it up: the way he counted 'by ten and by ten' is the way to count Kit's.
Harder
Start in the kitchen. North by one is the dining room. 'By one and by one' is two, as Holmes's 'by ten and by ten' was twenty paces: west by two is the hall and then the library, south by two is the darkroom and then the cellar stair. And so under: under the cellar floor.
The mark, in words
The plan of the ground floor as twelve buttons in three rows of four, with the kitchen already chosen as the start. Tap the rooms in the order the ritual steps them, then Step it.
The answer
The answer is c2-c1-b1-a1-a2-a3.
The fire that never went out is the kitchen's. North one to the dining room; west two, through the hall to the library; south two, through the darkroom to the cellar stair; and so under, as in the Musgrave Ritual, where Holmes found what he was looking for under a flagstone in the cellar. Kit's card is under the flagstone beneath his own trunk.