SidMo

Yes, I'd agee with the others. This is a decent tip for being in a pinch, but if you have an old house or work on them for others, you really ought to have a big bag of plaster washers. It's a whole lot less time consuming and they'll work better (bigger diameter, full of holes to key the skim plaster). I buy mine from Charles Street Supply (google) whose big quantity prices are a bit cheaper than Lee Valley. Seems like the Lee Valley price for 100 is good though.

Peder is dead-on in another regard too. I don't think you want the washers right on the fracture line! Rather you want to stagger them down both sides of crack an inch or so away from the actual crack in solid undamaged plaster. Holding both sides up with 1 smaller diameter homemade plaster washer right on the weak damaged area seems like a bad idea... Use a lot of real plaster washers to give solid support. You are reskimming anyhow -- make it last.