ATC SCM19 V2 Recommendations.

Equilateral triangle, as far from side walls as possible, use sound anchor stands (ADJ 2’s for variable height or the shorter 3 post fixed height model). That will get you started. Your iPhone has SPL meter apps available. Pink noise is useful for walk testing your space, you hear how the position changes the balance from bass to no bass, rolls off high end, boosts high end, etc. Very easy to hear how pink noise changes character with position to speakers vs those changes in music with position.

Remember it’s the direct vs reflected energy ratio that affects the sound of things. The more direct sound the better, the more reflected sound the worse. You get into a triangle small enough to raise the level of direct over reflected energy. Panels and treatment can expand the size of the triangle.

@tony1954   What are those stands?  Can you make them ring by tapping them lightly with a screwdriver or wrench? 

6 foot triangle works but how far from the front wall (the wall behind the speakers)

So glad to see you invest in the life's work of Billy Woodman.  That driver was I think his first SL driver design, came after the midrange.  The tweeter in that pair is a miniature version of the ATC mid design..  All hand built in the Cotswolds, not too far from the old Jaguar factory.

@tony1954 

Whatever sound the stands make, that is added to the speaker.  Mass is your friend in stands so the fact they are filled an don't resonate is good.  42" away sounds good.  Now how far from side walls?

So you are a Brit too!  Well if you ever go back, call them a week or two ahead and get an ATC factory tour.  It's fascinating because it's the way all the speaker makers used to look (winding coils, building drivers, etc ) but almost none look like that now.    Most buy all the parts/cabinets from a third party.