How do you get the % difference of the actual reading and graphical method in composition of forces?

Question

How do you get the % difference of the actual reading and graphical method in composition of forces?

Answer 1

You need to decide which one seems more trustworthy.  Then you take the difference in results and divide by the one you trust better, and multiply by 100%.

So, if you think your measurement of the "actual reading" is reasonably precise (i.e. your instrumentation is good), you'd divide the difference by that value.