I'm getting ready to pour a concrete patio which meets with an existing half-round concrete step. I have a good surface opposite the radius to run a screed board but I'm not sure about what to use on the outside radius for the screed board to ride. The radius area of the existing concrete step is about 6 feet wide. Also, about how long should I wait between screeding and floating the surface? It's a relatively small area,6'X20'. Any ideas? Thanks.