Look up the datasheet for the film and they will usually provide a reciprocity chart such as this one (I'm pretty sure this is for Tri-X 400)



Now meter your scene. Lets say your meter says you need to expose your film for 5 seconds. Well take your finger and place it on the number 5 (on the "Calculated Exposure Time" axis. Now move your finger up until you hit the curve. See where your finger is in reference to the vertical axis now (Adjusted Exposure Time).

So for example:

10 second exposure on your meter = you really need to expose for ~50 seconds
20 second exposure on your meter = you really need to expose for ~2 minutes


The problem is that with Ektar 100 there is no chart on the datasheet. Usually when I shoot Ektar at long exposures, this is what I used (found somewhere on the internet). You can curve fit these points to interpolate reciprocity for an hour long exposure.

Calculated Exposure Time = Adjusted Exposure Time
1 sec = 1 sec
2 sec = 2 sec
4 sec = 4 sec
8 sec = 8 sec
16 sec = 20 sec
32 sec = 40 sec
64 sec = 115 sec
2m 18s = 4m 16 s
4m 16s = 9m 23s