Rising air cools at about 9.8 °C per km (the dry adiabatic lapse rate) while the dew point falls about 1.8 °C per km. The two converge at roughly 8 °C per km, which gives the classic rule of thumb: cloudbase ≈ 125 m (≈ 400 ft) for every °C of spread between temperature and dew point.
Why the spread matters: temperature and dew point converge with height
Use the temperature and dew point at the time you expect to fly, ideally from a local sounding or station. This is an estimate of cloudbase above ground level; the actual base varies with airmass and terrain.