Weighing fertiliser.

I think it’s useful to weigh an amount and then see how that looks when you spread it on the appropriate area, just so you’re not widely out. After that you can just judge it by eye. If you overdo it, the excess will run off which is best avoided for financial and environmental reasons.

It depends . . . how big are your hands?  ( Kidding!!! )

There are a lot of variables.  Some plants perform poorly if they get too much nitrogen (okra is a crop I grow that does this; there are others as well).  It also depends on the formulation of the fertilizer (is the fertilizer a 30-10-10?  16-16-16? 4-18-38? Readily soluble, slow-release, or organic)? 

If you toss down a handful of fertilizer that is easily soluble, you run the risk of overdosing the nitrogen or some other component.  If the fertilizer is slower to release (example, blood meal) doing the same thing might be OK. 

To your question, will the plants just take up what they need?  It is more likely that plants will take up nutrients at the rate that they can, but that can be limited by a nutrient that is in short supply (a rate-limiting nutrient).  For example, if you are supplying nitrogen and potassium in adequate amounts, but not enough phosphorous, the phosphorous might limit the growth and production of a plant, no matter how much nitrogen and potassium are available.  This is called  Liebig's Law of The Minimum (or just Liebig's Law, there are many web references on this subject).

Would you be better off measuring how much your crops need?  Probably, yes, from the standpoint of under- or over-fertilizing.  Also, potential loss of fertilizer due to runoff if you apply more than you need.

There are charts available from many universities and government agriculture programs that give advice on fertilizer requirements.  Some seed companies also do this, for example:  https://www.ufseeds.com/fertilizer.html

Here is a university website that provides calculators for many application rate questions, including for various fertilizer formulations: https://aesl.ces.uga.edu/soil/fertcalc/

Some fertilizer recommendations are given in linear bed feet; more university information: https://nwdistrict.ifas.ufl.edu/phag/2016/08/26/using-the-linear-bed-foot-system-for-vegetable-fertilization/

Some fertilizers give garden fertilizer recommendations in either pounds per 1000 square feet or pound per foot of row.  A hypothetical fertilizer might come with a recommendation to apply 5 pounds per 1000 square feet. if you were wanting to just apply to a 1 foot square (around a pepper plant for example), that converts to 0.005 pounds per square foot (or 0.08 ounces, or 2.3 grams).  For the fertilizers you use, if you have a kitchen scale and some measuring spoons, you can probably work out a conversion chart (my hypothetical fertilizer would be between 1/4 to 1/2 teaspoon per square feet, if you assume 1.5 grams per 1/4 teaspoon).  Do you need to get that precise?  Maybe not, but the calculation shows that 23 grams might be too much for the same area.

For recommendations in pounds per linear foot, just remember that the linear foot is often considered as 1 foot wide.  A super phosphate fertilizer I have recommends 4 pound per 50 linear feet for bulbs (which sound like a lot to me, or maybe too much, in an area 1 foot wide and 50 feet long).

Sub plot. Is the Leibig you mention the same one who invented some sort of condenser or a similar piece of scientific equipment? Anyway, thanks for your comments. I guess I already knew the answer, but I have tailored the feed for the plants i.e. flowering = Q4, leeks = 6X.