A teleconvertor is simple...1.4x converter = -1EV of lens speed (f/4 indicated is really f/5.6); 2x converter = -2EV of lens speed (f/4 indicated is really f/8)
Tubes are more complex. It is determined by Extension/FL. Rules of thumb...
1/10 - 1/5 = add 1/2EV more exposure
1/4 - 1/3 = add 1EV more exposure
1/2 = add 1.5EV more exposure
1 = add 2EV more exposure
2 = add 3EV more exposure