So if you must find the X value greater than 10% in Z = (X - u) / σ. If you look up in the Z value table, there is the option 1.28 (0.1003) or 12.9 (0.0985), what if I want exactly 0.100, how would I calculate the Z value for this ..
Similarly if you're trying to find percentage for a Z value with 3 decimal places (table only shows 2), how would you go about this, the answer for this is here , but quite frankly I don't understand it (specifically how he/she gets the weighted sums o.3 and 0.7), surely there is a more concise explanation.
Thank you, let me know how I can improve on this question if neccessary.
If you want exactly $0.100$, the $Z$-value you want is very likely to be an irrational number, and hence not representable on a computer (or on a piece of paper, except indirectly, using a name like $\pi$ or $e$, or an unresolved computation, like $\arctan(1)$).
As for your "surely there is a more concise explanation," the explanation you point to is two sentences and about 200 characters!