How do you estimate necessary dynamic range?
Well, as you guessed, it depends!

For example, assume you're observing a multi-carrier communication signal. It has probability of 0.1% of symbol times to land in upper the percentile you "cut off".

Let's say that in that case, you get a bit error rate of 25% whenever signal is cut off.

Now, you get a BER –due to cutting off– of 2.5·10⁻³. That's pretty much for some systems, and nearly nothing for others.

So, define this from what your measurement describes, and generate a curve of "error you introduce" over "cutting off at a specific level"; let's call that function $e(l)$ ("error of level").

Then, calculate how often that level $l$ or anything lower occurs; that's a cumulative distribution function of the signal amplitude! It can be easily estimated from an observation. Because it's a probability distribution, we'll call it $F_L(l)$ of the random variable $L$, the observed level.

$\tilde F_L(l) = 1 - F_L(l)$ is just the "counterpart": How often does a value above $l$ occur?

Now, you can find the cumulative error of a cutoff-level by just defining a function 

$$C(l) = \tilde F_L(l) e(l)$$

and evaluate it for some different $l$. (Do a plot of this, really.)

Then, calculate the percentage of broken measurements you're willing to accept, and compare your $C(l)$ to it.