Mike

Cassandra got it right on the money; your table should be 3-5" below your elbows (when standing) for the average person. This allows for good control without stooping or bending. If you are building a sit down table I suggest you follow this process: Get a good swivel chair on casters and adjust the height so your legs are level. This will provide maximum comfort. Set your hand on your knee as if you were reaching to shake hands with your thumb laid on your fingers. Add an inch for clearance to the table bottom. The reason for this is to allow easy access for adjusting your router. If you are building a sit down version of the Router Workshop table or using other brands of bench top routers then make your height one inch above your legs to the 3/4" thick bottom support.

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Mike