There is one trick I use with my FM2n & FE2.
For example, say you are using 100ISO film. And TTL metering tells you that at maximum aperture, and at 1sec, it is still underexposed. What you do is that you turn the ISO knob to 800 and check the metering. If it's still under, turn it to 1600, check. Still under? Repeat. What this is is that it gives a rough guide on how many stops are you under, so you can inturn calculate how much time you need to expose for.
Now let's say you get a correct metering at 3200. 100ISO -> 3200ISO is 5 stops (of light) difference. Hence meaning, time for exposure, calculating from 1sec, is 30secs.
1sec -> 2sec -> 4sec -> 8sec -> 15sec -> 30sec
(1stop) (2stop) (3stop) (4stop) (5stop)
This gives a rough estimate of the correct exposure. But actually if you are shooting at very long exposures, you don't really need to time down to the last second. Make sure you bracketing too, although you should already be bracketing most of what you shoot.