Don't just pick an arbitrary number that's a percentage higher then published VFR minimums. Get a real perspective to what distances and cloud bases actually are and look like. Take your map and sit down in a location that has a lot of surrounding identifiable features like hills, mountain ranges, radio towers, towns etc. and estimate how many miles you think they are and if you would feel comfortable taking off from your position if they were obscured . Then measure them on a map, you'll be surprised just how short a distance 3 miles is and that's sitting on your arse not traveling at 100kts with the weather perhaps shortening distances. Then start figuring the cloud bases, if you have local airfields that have ATIS information via telephone, call up on different days when the weather is overcast at different levels to get a picture in your minds eye what 1500, 2000 feet etc. looks like in relationship to the terrain features around your local flying area. Listening to forecasts is one thing but being able to assess distance and heights in a realtime situation is whole different kettle of fish. Best to bone up on those skills sitting on the side of a hill with a map and flask of tea than in the air with a dry throat and white knuckles. You can learn an awful lot about flying from the safety of terra firma.