Hm...I don't see need for mangle here. You can just do simple queue and place it on top of other queues
go to queue simple and type this:
add name="site" target-addresses=0.0.0.0/0 dst-address=x.x.x.x/32 (site IP)interface=all parent=none priority=8 queue=default/default limit-at=0/0 max-limit=64000/64000 (or whatever) total-queue=default
And this should limit all traffic to specific site to 64k
Remeber, this queue should be placed on top of other queues.
I drink like a pirate and smoke like a hippie...