Bottleneck calculators are complete nonsense and any percentages they provide should largely be ignored. In reality, some games are more demanding on the CPU than others, and even the fastest CPUs on the market today will limit performance in at least some games, especially when paired with a higher-end graphics card running at lower resolutions. With an i5-7400, performance per core is arguably decent enough, though only having 4-cores without SMT is likely to result in less-stable performance in some of the more heavily-multithreaded games.
If you are planning on buying a GTX 1080 though, pricing is not exactly all that great compared to newer cards, at least in the US. A current-generation RTX 2060 will typically be nearly as...