There isn't a working PSU calculator. But a high quality 400W is already enough. Peak gaming power consumption with a stock GTX 980 Ti is around 300W DC, with a overclocked model it could be around 350W DC. But the SF450 is enough with every GTX 980 Ti out there as long as you don't use some extreme overclocks.

And yes it's that quiet and that good.
http://www.tomshardware.com/reviews/...ly,4512-4.html

Every other SFX PSU is just the wrong choice.