If the camera has through the lens metering, then the meter will give the correct reading through the tubes and lens combination. If you’re using a hand held meter, then a calculation is needed.

Rather than bore you with the calculations,

with 50mm lens at the “infinity” setting, you get the following results in the form:

Tube or combination of tubes, time multiplication factor, number of additional f-stops required.

10mm, 1.44x, 0.53f

17mm, 1.80x, 0.84f

31mm, 2.62x, 1.39f

(10mm + 17mm), 2.37x, 1.24f

(10mm + 31mm), 3.31x, 1.73f

(17mm + 31mm), 3.84x, 1.94f

(10mm + 17mm + 31mm), 4.67x, 2.22f

If you focus the lens closer, the exposure requirements increase slightly, but not by much for a 50/1.8 normal lens with its relatively short maximum extension.