I hacked the nestedSlidersPlugin a little by adding the 2 lines at the end in this part of it:
...
if (place.style.position!="relative") {
var left=findPosX(btn);
var top=findPosY(btn)+btn.offsetHeight;
var p=place; while (p && !hasClass(p,'floatingPanel')) p=p.parentNode;
if (p) { left-=findPosX(p); top-=findPosY(p); }
if (left+panelWidth>rightEdge) left=rightEdge-panelWidth;
if (left<0) left=0;
var mod=top/1.005;
top=top-mod+90;
}
which gets the floating panel in the ball park near the button regardless of how many other tiddlers are open now, except for it doesn't work well when the button is low down within the tiddler (as opposed to low in the page)
I'll keep looking - maybe there's a variable that indicates the height of the current tiddler I can throw into the formula...