"On another point - when you say it doesn't matter if you assign the intervals to the step that ends them or the step that starts them - i think perhaps you didn't mean to say this. When we say a 'slow' we surely mean that the interval to the following step is a slow - not the interval from the previous step."
No, I really meant that it doesn't matter - because neither one of these intervals is the slow. Instead, the interal between feet passing is the slow. When we speak of intervals between step placing/weighting, assigning them to the previous step vs assigning them to the next step is purely a matter of personal opinion, because officially they straddle the boundary between steps.
"I agree that the 'official 'steps start when the feet pass, that's why these sorts of discussions can be very confusing. However, i generally think (and measure from a video if i'm doing this)of 'steps' starting from the placement of the foot."
Yes, most people initialy do that - but I hope you are starting to see why it's counterproductive in foxtrot. I mean really, are you going to time your steps to a fraction of beat requiring two decimal places? No - you are going to time the overall movement of your body to match the overall progress of the music.
"There's been this discussion before but all the slow waltz videos (and dance competitors) i've eve watched placed their feet on the beats. i know foxtrot is a subtle dance and it's not so simple as this."
Exactly - foxtrot is not waltz. Or maybe we should say, foxtrot music is not waltz music. The steps are much more similar between the dances than the music is. A plausible model (though not a very good one) is to say that we dance waltz's three steps against foxtrot's for beats. That's not really right... but its probably more accurate than trying to literally interpret SQQ would be.
"One thing though is for sure - i don't normally place my steps exactly on the beats in foxtrot when i'm dancing well. We start our comp routine with the classic feather - reverse turn - feather finish - 3 step - natural turn and in all of those figures i'm getting onto the first quick (nominally beat 3) about half a beat early. It feels right that way."
Well, I'd recommend you try to hit that third beat right one as doing that is rather common advice. To do it, you will probably need to draw out your second quick so that you are starting your initial slows later. Wheras most people waist a lifetime starting the slow too early and then trying to wait for the music to catch up to them so that they can put the first quick on beat three. Don't get ahead of the music and wait for it to catch up to you, let yourself get "fashionably late" so that you can drive to catch up to the music.