Remember that you will not collecting the measurement voltage from the
fuse, but from the fuse holder, so, whenever you change
the fuse, or some temperature contracts or expands the fuse, the
resistance between the fuse and fuse holder contacts will change
considerabily, creating problems for you.  It would be wise to read the
voltage from the fuse itself, but it is a little bit difficult.

Also, I believe the fuse changes its resistance according to temperature
as any other conductor, so your reading will be a non linear voltage.
As the main function of the fuse is just create heat, even using a 20A
fuse into a 7A circuit, it will get a little warm. Changes in the
ambient temperature can create more troubles to you.

If you never did that, measure a resistance of a 10k Ohms common
resistor while touching its lead with a hot solder iron tip, and take a
look what happens with its resistance.  You can say it is an almost
perfect thermometer...

Measuring current is one of those nasty electronic jobs, because you
need to open the circuit and introduce lots of problems and connections,
read here "resistances and voltage drops".  If you can find a solution
that not involves opening the actual circuit, it would be nice, like for
example measuring the voltage drop in some cable alctually installed.

If you really need to introduce something in the circuit, there are
calibrated current shunts, a flat piece of thin metal, 1" by 1/4", one
hole in each side, generates 50mV for some specific currents, as 30A,
20A, 15A, and so on.  I can try to find where you can buy it.  They are
widelly used as Shunts in analog Amper-meters around.

Wagner