It might be overkill for what you're doing, but the Orca3D plug-in computes the volume of surfaces, meshes, or solids at any arbitrary plane. You can set the volume units to just about anything (gallons, barrels, liters, cubic feet,…"
"Here's a Rhinoscript that will do this using two rail curves. Drop it on an open instance of Rhino to test. You can add it to a button or make an alias for it if it seems useful. Not heavily tested. Let me know if it…"
"Hi Ryan - if you mean fillet a corner of just a curved surface that has no thickness, no there is no direct way to do that. One way out is to BlendCrv between the edge curves and use the resulting curve to trim the surface. Is that what you are…"