Measuring is good when you're trying to get your proportions right, but in the end it will really come down to your shading. Always remember reflected light, and that shadows are not hard edged blobs of dark, but rather they fade out around the edges. And when using color remember that shadows aren't grey or black, but more of a mixture of the color of the surface they're on and that colors main compliment. Outside they're usually somewhat purple. It's strange, but I've noticed this to be true.
Both proportion and shading can be improved at the same time by getting yourself to some life drawing classes. They are honestly one of the best things you can do for self improvement. Alternatively go out and sketch the world around you, it doesnt matter what it is, everything has proportion and shading so you cant lose.
when I was really young I used to just copy out brand names and fonts freehand. (The coke logo was always my favorite) I could do this a lot in class or when I was out and about, on napkins and scraps of paper just simply out of boredom. Now I realise that from that I learnt how to approximate proportions and pick out the exact shape of an object very easily in my head.
I think if you practice simply copying out common images and logos, patterns (or doodling!) when you get the chance, not in a nice sketchbook but on scraps of paper on a bus or in a coffee shop then maybe you can practice your skills without the pressure of the drawing being good or not. You're going to throw it away or lose it or use it as a bookmark later! If you wish to keep it and look back on it, make sure you store them away and don't look at them all the time. Only then do you see a real difference and improvement in your work.
All artists need to learn when to distance yourself from your work and not take it too seriously then you will start drawing for the fun of it, as a necessity to occupy your wandering mind.
And here are a few basic guidelines for proportions within the face I have picked up through the years; Face lines apply for a neutral expression, and warp as the expression changes; -The pupils always line up with the corners of the mouth -Inner corners of the eyes line up with the outer corners of the nose -diagonal line drawn from the outer edge of the nostril up through the pupil dictates where the highest point of the arch of the brow bone should be -bottom of the nose lines up with the earlobes
Eyes; -There is always a highlight somewhere on the eyeball -Every eyelash is individual and different -crows feet are accentuated when the subject is smiling -Pay attention to the exact shape and slant of the eyes, a fraction of an angle can be the difference between someone being recognizable, or looking sick. These guidelines are only baseline. Every face is different. They key is to be able to pick out the tiny details where these guidelines are broken.
Body; -Men's hands are generally more boxier and broader than a woman's hands -the fore-arm is the same length and the foot. (weird I know but measure on yourself!) -look at some of Leonardo Da Vinci's studies of the human form, they really lay out what should be obvious but often goes un-noticed.
Shading is just like glorified coloring in; treat yourself to a nice set of graphite pencils and experiment with those. Shading is very easy to get a hang of.
I don't know what tutorials instructed you to measure your drawings, but I did that for years with no real progress. I've seen people who do get good at copying reality this way, but to me the results always feel cold, clinical and detached. There's no expression or emotion in them, typically.
Instead of measuring, draw the shapes found within the figure, pay attention to their relative sizes, how they overlap.
If you're still a beginner though, concentrate first on finding the lines of rhythm within the figure. Lines which do not hug the outer contour but which bounce back and forth from front to back, left to right. This is what connects all the 'parts' of the figure into one whole thing. Then I'd work on finding surface contours, again these are lines that follow the dimensional surface rather than the flat silhouette.
Once you get past mastering these, then go back to shape and start seeing how shapes on the page represent real volumes that you can trace in your mind as if the pencil is moving into and out of the surface. It's being able to trick your mind into believing that's what happens even though you're drawing on flat paper.
I leave a link in my signature for referring people to the source where I picked these things up.