Hi,
Pardon if this is basic or obvious to some of you,
but in reducing my boils down to a specific volume I always end up
worried and anxious near the end.
Say I am shooting for a volume of 400ml in my pot. I start reducing,
but when I get near what looks like 400ml I am always worried about overshooting and
burning the product. I admit I am somewhat lazy and don't like to just hover watching the pot boil.

So I hit on this solution:

Get some wooden dowels. Old fashioned wooded yardsticks would work too but I haven't seen
them around in years. or you could use wooden kitchen spoons.

Before starting the boil, measure the amount of liquid that is your final goal, eg 400ml, of water and put it in your boiling pot. Put it on the burner but don't turn it on. Let the water settle. Touch the dowel to the bottom of the pot. Now take a piece of tape and mark the line where the water came to. Let's call that depth A.

Next pour out the water and pour in the liquid you want to reduce. Bring it to a boil then turn the burner off and let it settle and level out. Take another dowel and again mark the depth on the dowel. This is depth B. Turn the burner on and boil for say 2 hours. Measure the time exactly. At 2 hours, turn the burner off and let it settle. Use the second dowel and measure the depth again. Mark this as depth C.

Now using a ruler measure the distance on the dowel from depth B to depth C. The rate R at which your water level is going down per hour is B-C/2 So say it dropped 1.5 inches in 2 hours, the rate is .75 inches per hour.

Great. Now measure C-A. Call that DR, That is the depth remaining to boil off . Let's say it's 3.5 inches.
So the calculated time remaining to your target depth is DepthRemaining or DR over Rate or R

DR/R = 3.5/.75 = 4.666

So you should be done boiling in approximately 4.7 hrs.

Set your timer for say, 4 hrs and 20 mins. Check the depth with your first marked dowel, again with the burner off and the water level still. When the water level matches your mark, you're done.

Hope this is usefull to someone.

I do something similar to this except with a ruler and no times. I just measure the desired amount with a metal ruler, pour out the water, fill up with juice, heat and measure occasionally.

I tend to be around the kitchen when I cook so I don’t pay any attention to time.