''Boil the mixture until it reduces almost in half...''

I hope you get some good answers to this, because it drives me nuts!

Sometimes I use a cheap chopstick: I dip it in and draw a pencil line at the beginning level. But there has to be a better way.

That's good enough for me. I've a bundle those bamboo skewers one uses for kabobs. I been looking for a use for them.

Just seeing the word "almost" makes it pretty clear to me that this is not a precise, scientific measurement. Just eyeball it. OR stick in handle of wooden spoon you're stirring with. As soon as I finished typing that line, decided gellchom had the BEST idea... at least so far.

This is the kind of thing women have been doing from long before we had measuring implements: Rise until doubled, reduce (steam off) liquid by half, divide into thirds. "Eyeballing" it means looking for reference points. if my pan is reasonably straight sided, I look at its level, and figure where half way down should be and call that my reduction point. With angle sided pans, I guesstimate more of a 1/3 the way down.

I think when you've cooked enough, you just learn what it's supposed to be like when reduced. That is, watery, brothy, syrupy, or gellish. I used to make a lot of sauces that would reduce too far before they were otherwise cooked, so I'd add back some wine or stock or whatever was called for and let it get to reducing again.

So that's all about why the recipe says that. Your skewer with Gellchom's suggestion should do the trick until you're ready to start eyeballing and trusting your cook's instincts.

Any time a recipe calls for an onion or a bunch of parsley, three tomatoes, 4 chicken thighs, you know that 4 or 5 minutes more or less cooking a sauce down will not make much difference at all.

Akin to this problem is when a recipe says to pour half the batter into the pan. The batter is in a round bowl. I've never figured out how to figure half a bowl of something.

Sally

If I am making something with a lot of volume (like a cake) to divide in half, I put one cake pan on my flat top scale and tare. Then pour what you think is half and note the grams or ounces, do the same for the other pan and add a little to the one that weighs less until your measurements are equal for both pans.

As for reduction, it is an eyeball thing. For a small amount I usually go with some sort of thickening look. It usually is not critical for the final result.