How should I calculate the error in my e/m ratio calculation for my homework?

[Phystudent91]

Homework Statement

I'm in the middle of a project finding the e/m ratio of the electron by the use of the formula below. I know all required values and can find a result, I'm unsure how to calculate an error for the calculation.
In the formula below, D, n, A and f are all given quantities (therefore no error/the error can be taken as 0, right?)

Homework Equations

$\frac{e}{m}$ = (32/D²)*(nA2∏)²*(V_a/I_s²)*(I_s/V_s)²

The Attempt at a Solution

The last 2 parts of the equation are where the trouble arises. I have had to measure and record results in two different ways to find values and to plot the 2 graphs - V_a/I_s² and I_s/V_s - and the software used gives a percentage error in the line.
Do I need to find the error in each measurement for V_a and I_s etc?

The current way I know to calculate error would mean using the smallest value measured and half of the smallest possible measurement (half the equipment increment). This would result in a stupidly high error, eg:
smallest V measurement was 5V, this is also the smallest increment on the equipment, therefore error is ±2.5V meaning an error in that set of measurements of 50%??
I'm then taught to add the percentage errors... this leads to 350% error.
However, because I know the gradients of the graph, is it acceptable to only use these errors? This would give a total are of ≈10%. Much better!

Which is the correct way of going about this? And if the answer is neither of these, what is the correct way of going about this!?

Thanks in advance!

 

[Phystudent91]

Any Help would be appreciated!