It is easy to route using the SG300 switches. Setup the SG300 switch in L3 mode. Do not use a trunk port to the router or the router will end up doing the routing. Use an access port on the SG300 switch pointing to your router. The router will need static route statements on your router pointing to the SG300 switch port connected to the router. The SG300 switch will need it's default gateway pointing to the router. This is all there is to it.
I created a VLAN on my SG300 switch with 1 access port to connect to my router. I call it a router VLAN. I use a 30 bit mask.
Just to be clear you do not need to define the network VLANs on the router which exist on the L3 switch as the L3 switch is doing the routing.