Rather than looking purely at numbers, look at what conenctors you need also. Like if you're using a graphics card which needs 2x6-pin PCI-E power. A quality power supply maker won't put 2x6-pin PCI-E on a PSU unless they are confident it can reliably push the full 150W combined down them. While you might
be able to use a 300W PSU OK for this, they'll take a conservative approach and Seasonic will put 2x6-pin PCI-E connectors only on their 450W models upwards. Likewise they'll be sensible with numbers of SATA and molex connectors based upon power.
So, if a quality PSU has enought connectors to hook up to everything you have, you'll be fine. The TDP calculations will back this up but the manufacturer will already have done this to spec numbers of connectors.