how will i be able to know what % to put or what number to put

Kevin,

There are no right or wrong percentages or pixel values, per se; it all depends on the needs of a given site's design! There are some values that you could find that are recommended for certain heights/widths/spacing, etc. to fit particular screen sizes or average touch sizes for mobile and things like that, but ultimately the pixel and percentage values are whatever you need/want them to be. The 940px max-width that Nick applies to the main wrapper of the site is a fairly standard size, or at least is in the ballpark of what could be considered "standard", but really it's just the value that works for the design of the site that he's building. Any "standard" values you might come across online are made over time based on "standard" or "typical" screen sizes. But, as always, screen sizes are changing, and thus, so are "standard" values for widths. There isn't necessarily a "right" size that you can learn or "know", it really just depends on your design. If you want your site to be the full width of any given viewport, well, you set your values to be 100%. If you know, though, that your design will start to suffer and break when it gets wider than 1000px (by setting and testing various values in your local development environment as you build your site), then that's how you land at 1000px as your max-width value. It's really that simple: Trial and error, on a site-to-site, design-to-design, section-to-section basis. Percentage values for things like the portfolio images come from Nick knowing that he wants images to be able to sit next to each other, two in row, so you translate that to mean "they both need to be about 50% of the width of their containing element, because two pieces fitting in one whole is equal to two halves, and half of 100% is 50%." From there, you just subtract your percentage widths a little bit to allow for some spacing around the images so they look nicer, so you land at 45% width. There is no law or rule or anything that you'll learn, really. It's just whatever works! You try one value, and if you see that it's too big or too small, you adjust until you find the right one. For these videos, they have their sites and designs planned ahead of time, so they "just know" the exact values to use. But in a normal situation, you will be trying different values until you find the one that works!

Hope this helps to clarify a little bit.

Erik