If one looks at the formula for volt it has a lot of items which really do very little.
I have also tried to work it out and for years used an Excel program. New phone needs for me to pay for Excel so Java Script seemed a better idea.
I was helped with
this calculator by people on here and it seems I had made a mistake with original which only came to light as I moved it to Java Script.
I am sure there are better calculators out there I used method 100 with my calculator and clearly with other installation methods there will be errors.
However to design an installation to run right on the limits would be rather fool hardy and it does do a near enough job for me.
Some of the variables do seem questionable "Max permitted operating temp tp" will alter the "Correction factor Ct" which in turn will alter volt drop. I know this is the formula given in BS7671 but personally I just can't see how using different insulation on a cable can mean the volt drop changes?
Since building the calculator I have not done enough to really test it the point is of course we want to ensure once complete and we test and inspect that it will fall into the laid out limits.
However if on the edge then the next question is how accurate are our testers. Take any tester and measure PSCC three times and each result will differ. 10 amp on the PSCC can mean 1 volt extra or less on volt drop and that is both for the source and destination so best will in the world if the volt drop is calculated at being 2 volts over it would be hard to say if that was due to measurement error or design error I would not want to fail an installation if just 2 volts over the limit.
As to loading pick up a laptop power supply and it will say something like 2.4A yet I have seen school laptop charging trolleys with 20 laptops being charged from a single 13A lead without blowing the fuse. As you say switch mode power supplies cause some odd results.