Games fluctuate fps constantly, there's a massive difference in what it takes for the cpu to place all the particles in an explosive fireball or a field of grass, vs staring at a brick wall.
None of the AC titles has been spectacularly optimized, AC3 was really bad for frame changes.
The cpu sets fps, not the gpu. The cpu isn't affected by detail settings or post processing affects or resolution. That's all on the gpu. As is, the 1070ti is strong enough to max out all the frames the 8400 can give it at any detail level, so you'll not see much of an fps change. It's when adding stuff like physX or hairworks that the gpu takes a big hit, so fps drops, but that doesn't affect the fps the gpu receives, just the fps it can put on screen.
Play the game, decide for yourself if it's playable or if you need better. Do not rely on calculators or website benchmarks, they are not accurate to your particular pc, and get endorsed by companies trying to sell you stuff you don't need. There's more than likely an ad or link to a psu or motherboard/cpu combo or gpu saying that's what's recommended to fix your problem.