Most household bulbs don't have adequate inside frosting to work well as an enlarging bulb. I've occasionally found exceptions. Also, an enlarging lamp burns hotter and thus is more efficient. Its higher temperature (and color) may slightly affect the use of variable contrast filters. As for the ideal height of the bulb, you can determine that best by trying it. When contact printing with an enlarger, it's a good idea to close the lens down a stop for more even illumination.