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Feed rate formula for milling

In my experience as a machinist, this topic of discussion is always confusing because I can ask 3 different machinist on how they calculate their feeds and speeds and I can get 3 different answers. At my shop, I am finally doing set-ups which is great, fun, and I'm getting a lot of experience, but when it comes to trying to tune my cutters for them to last longer it's a struggle. How do you guys calculate your feeds and speeds? I was cutting titanium today and my lead happened to pass by and I asked him what is the formula, but he said there really isn't a formula and it's all about the experience and noise it makes. Now I believe this to be true but there has to be something to get you close to what you want. I asked another machinist and gave me his formula which was (4XCS/ tool diameter). So what is the best way to calculate/approach it? Especially on tough materials like heat treated titanium/ heat treated inconnel or even aluminum. Looking forward for great answers.