The further away the lamp, the more light required, so exposures need to take this into account. The main advantage to using light integrators instead of timing devices to measure exposure is that they measure the amount of light hitting the stencil, and automatically compensate for different distance or variations in light source intensity. When using a timer, it is necessary to test expose from specific distances using an exposure calculator, and then use these test results to determine different times for different distances. Also note that point light sources expose the stencil quicker directly in front of the bulb, with the areas to the outside taking longer to expose. Under-exposure , or not enough light, results in thin stencils, pinholes, delamination of the stencil during printing, and breakdowns on the press.
Stencils are also harder to reclaim if they are underexposed.
Remember, the light is working its way through the emulsion layer from the back facing the light, through to the top side of the screen. This is why underexposed screens are sticky or give off unexposed emulsion scum or slime when rubbed or blotted on the squeegee side of the screen. This is also why you have to keep washing the whole area of the stencil even after it looks like it's finished. The last bits of unexposed emulsion will dry on the top, and then when you start printing they will rewet and fill in your stencil. Over-exposure will result in fine detail filling in, and poor washout.