My experience with long exposure and reciprocity is that you can hardly overexpose. So if you take a ballpark compensation figure, just add some more to it and shoot that.
Also, at first I was always watching the clock/timer with anticipation etc.. But now I know that a minute difference doesn't make that much difference for a 7 minutes exposure, and so on...
For my favourite film (Shanghai) reciprocity seems to match exactly the fp4 formula : T' = T ^ 1.48. I use a derivative of that I can calculate in my head in the field, but it's probably only useful for computer trained people :
Example:
T = 18 seconds
+ I take the closest power of two to T, that is 16
+ 16 is 2 ^ 4 : 4 is this thus the number of half/stop to compensate, or, two stops.
+ So T' = ((18 * 2) * 2) = 72 seconds; and strangely enough, 18 ^ 1.48 = 72.078
Not bad
For the non-computer people, you can figure out which power of two to use by counting on your fingers 2,4,8,16,32,64 for 1,2,3,4,5,6 half stops.
With that method, I get pretty close without too much trouble :
T = 60 -> 64 -> 2^6 -> 6 * 1/2 stops -> 3 stops -> 480 seconds
T = 60 ^ 1.48 = 428 seconds...
Again, not bad, and on the good side or the error margin too (more light in)