A good-enough version of good-enough? When should I stop iterating?
I did not check the code used in Abelson and Sussman's book or wiki.
My guess is that the recursive (or iterative) procedure will somehow
organize to compute improve(guess,x) only once. In a recursive
procedure, you just compare the formal parameter which is the previous
guess with the actual value (current guess) that you would pass for a
recursive call, in order to decide whether actually do the call rather
than return it. There may be a difference of one (wasted) iteration
depending on how you do the test.

Regarding the fact that you do the test on $x$ or on $\sqrt x$, or
rather $\mathtt{guess}^2$ or $\mathtt{guess}$, that does not give you the same
precison if you look for the same difference.

One way of explaining it is that $|\mathtt{guess}^2-x|=
|\mathtt{guess}^2-{\sqrt x}^2| = |\mathtt{guess}-\sqrt
x|(\mathtt{guess}+\sqrt x)$
since the second factor has to be positive (at least when relative
imprecision gets small).

Since $\mathtt{guess}$ is supposed to be very close to $\sqrt x$, we know that
$\mathtt{guess}+\sqrt x$ is very close to $2\sqrt x$ or
$2\;\mathtt{guess}$.

Hence $|\mathtt{guess}^2-x|$ is close to $2\sqrt
x\;(\mathtt{guess}-\sqrt x)$, so that the condition
$|\mathtt{guess}^2-x| &lt; 0.001\;x$ can be replaced by
$2\sqrt x\;|\mathtt{guess}-\sqrt x| &lt; 0.001\;x$, i.e.
$|\mathtt{guess}-\sqrt x| &lt; 0.0005\sqrt x$
or, still using the same approximation:
$|\mathtt{guess}-\sqrt x| &lt; 0.0005\;\mathtt{guess}$

This can be further formalized, but the above informal reasonning
gives you the idea.

However, I did not check how fast this conputation is converging, and
what is the difference between $|\mathtt{improve(guess,
x)-\mathtt{guess}}|\;$ and $|\mathtt{guess}-\sqrt x|$, i.e. how fast one
gets closer to the exact answer with each new iteration (assuming the
precision of the arithmetics is still good). Their ratio should also be accounted for. It depends on the formula used for computing the approximation, which I did not look at.