OK thanks, it seems like labels are the way to go, eg:
tl.add("circlestart",0);
tl.to("#square", 6, {scale:2})
.to("#circle", 1, {x:400}, "circlestart")
.to("#circle", 1, {y:30}, "circlestart+=1")
.to("#circle", 1, {x:0}, "circlestart+=2")
.to("#circle", 1, {y:0}, "circlestart+=3")
However, I would like the circles to 'Chain' normally, following each other sequentially without having to add the correct insertion point.
Eg, so I would like to achieve something like:
tl.add("circlestart",0);
tl.to("#square", 6, {scale:2})
.to("#circle", 1, {x:400}, "circlestart")
.to("#circle", 1, {y:30}, "circlestart+=1")
.to("#circle", 1, {x:0}, "circlestart+=1")
.to("#circle", 1, {y:0}, "circlestart+=1")