Sorry Rassis - to answer fully :-

For reporting purposes, I categories the values to brackets as above, but to get a proper curve or smooth graph, I had to fall back to the original data. It took me a bit to realise why my data was a bit skewed.

And yes, as you indicated, NormInv was exactly the function I needed, provided I got the mean and std dev from the raw data first.

Was a lot easier than I imagined.

Thanks.