What is a reliable way to calculate actual (web) page loadtime

Try Yahoo's YSLOW, it will answer your post of the questions, but it works only with FireFox (actually its a plugin for firebug)

FireBug has a "network timing mode" where you can see how long it took to download each resource which makes up your web page.

Plus you should measure the time your server needs to prepare the request. Since you can't influence the browser and the network, rendering time on your server should be as small as possible.

Firebug is a great resource for this and loads of other information about your page loads. Additionally, Firebug with YSlow goes one step further. YSlow has a hadnful of checks that it runs against your page and grades it's performance based on certain rules (are you using a CDN, is your CSS and JS compressed, etc.). I've found it invaluable to make some major improvements (JS compression is a great one) to my sites.

http://www.webpagetest.org/ is an excellent resource for measuring load time

Also google chorme dev tools has a Timeline panel where you can record events, Here is a 2.5minute video showing you how Timeline in google chrome works http://www.youtube.com/watch?v=RhaWYQ44WEc