We continued working with 'spot' after Dusty picked it up so quickly. When I started to ask for duration, waiting a little before I clicked, she dropped into a down position. I'm happy with this.
Today my criteria is going to whatever target I indicate (mouse pad, bathmat, folded dog rug, crate pad) and waiting in a down for 5 - 10 seconds and I'm rewarding with a thrown toy to keep her focused out in front and to build drive from the target.
I realised that I don't often follow a click with a toy rather than food, so I'll try doing some more of that - pairing the click with a wider range of reinforcers. I have tried to get her playing a lot of different games with all sorts of things. Thommo I allowed to become obsessive over a tennis ball - he doesn't even like other sorts of balls.