Good. I agree that getting some data with greater resolution is essential. If you try to use higher order models on data where the largest part is due to round-off, you will get a mess. That being said, the suggestion of using data i-1 and i+1 to estimate i would double the true change of the function value and make the round-off half as significant. (It's also a better way to estimate the slope at point i.) Likewise, using i-2 and i+2 would improve things by a larger factor. But if you carry that to the extreme, you will miss local changes in the function.I think it is better to obtain more precise data first. I am using high energy lasers so using current modulation I should be able to lower the power and use a very sensitive meter.