Counting/Measuring Heads Quickly with Your Mind
If I showed you a picture of a model posing and asked you - "How many heads of theirs does it take to make up their total height?" - for experienced artists I'm sure it'd be easy for them to give a close approximate answer, but for me, I'm just a beginner so it's not easy to tell at first glance.
So I decided to come up with quick way to do it, and this is what I came up with.
Typically, you'd count the first head, then try imagine the second head, the third, fourth etc until you get towards the bottom of feet - This method of thinking works but it felt too slow for me, especially when it came to do 30sec or 1min gesture drawings.
What I realised was, I didn't actually need to try to imagine and count each head sequentially from 1 to whatever. But instead, I could just imagine every odd numbered head - (1, 3, 5 etc).
For example, if I asked you to draw a circle, then draw the same sized circle under it BUT have it the same sized circle away, I'm pretty sure you'd be able to do it pretty accurately without needing a ruler. (We're actually really good at filling gaps with our brain.)
Figuring this out has helped me a lot, and I hope it helps you too. Sorry if it's not explained well, but I hope this picture kinda makes sense.
TL;DR: Simplifying your thinking by removing unnecessary information. In this case it was imagining the odd numbered heads as a rough measuring guide. E.g. Imagining the 1st, 3rd, 5th, 7th head instead of 1-7.