In selecting a meter for low light you should pick one on how low light is capable of not how long the exposure time can be set. Among the low light meter, like someone has said the Gossen Luna Pro can go down to EV-4@ISO100. The Minolta booster unit can get my Flashmeter III to read down to EV-6@ISO100 but with an angle of 60 degrees. Of course, with long exposure time there is the reciprocity failure problem which there is no meter that would handle this automatically.
Once you have the meter reading the calculation can be done manually with the same accuracy.