steady state error

When we have to plot steady state error versus the proportional gain, how do we find the error in the steady state error.

For the model, the uncertainty will be due to the error in the open loop gain K. For the experimental curve, the uncertainty will be from precision error (you are taking an average to determine the steady state speed from your plot, so this introduces precision error).

Alright thank you. Also what exactly do we need to say when we compare the model to the experimental data?

You basically just need to make a comment about what range of proportional gains where the 1st order model assumption is valid (by looking at how close the model matches the experimental data). Obviously at very low gains, the model and experimental will be close together, whereas they won't match at all for higher gains.

Alright so just to clarify, do the points that are very close to the model imply that it's a valid first order system?

In general, the better that any model matches the experiments, the better that model describes the experimental system. Your assessment of the valid range of proportional gains where the 1st order model you have can capture the dynamics of the system will be where the modeled and experimental values match up better (if the dynamics of the experiment are truly 1st order, then the model should be able to capture that and match the experimental data).