1 second metered give 1 second.
4 seconds metered give 5 seconds.
15 seconds metered give 20 seconds.
30 seconds metered give 45 seconds.
2 minutes metered give 4 minutes.
The problem with compensating reciprocity failure is that the highlights don't fail but the shadows do. The longer the exposure the further apart the highlights and shadows get. This means a severe increase in contrast . A partial solution is to give an enormously extended exposure so the shadows register and then do a very contracted development so the highlights don't block. The experiments necessary to define all of this are tedious to say the least.