Note that with fps time code, the "second" counter is still increased after 24 frames, even though 24 frames add up to slightly more than a real second. When working with other time sources, confusion can arise since a "second" in fps time code notation is slightly longer than a real second, it is ..etc (24 × 1/) seconds long. In other words, when a time code was reached of 00:16:40:00, this does not mean the video has played for 16 real minutes and 40 real seconds (1000 seconds), but that it has actually played for exactly 16 minutes and 41 seconds (1001 seconds).
Bear in mind that the only way discounts make sense if working with the given CAT tool actually increases your productivity. In other words, if you can process more volume by using the tool, then you can think about offering some discount. The amount of discount should be proportional to the productivity gains, and that depends on the languages involved, the type of text and some other factors. For example, if the target language uses conjugations, inflections while the source uses prefixes, a small change in the source may require a more extensive change in the target. In other words, a source sentence that is a 95% match may be only a 50% or lower match in target terms. Sometimes the surrounding sentences need to be modified as well, even though the source is the same. Based on experience, translators can estimate the time savings/productivity increases that can be achieved by using a certain tool for a certain type of job, and may be able to offer some discounts after taking into consideration the cost of acquiring and maintaining the CAT tool in question.