HP 54710A User's Reference Manual page 208

Measurements
Measuring time intervals
Automatic Measurements
Except in special cases, the simplest and most accurate way to measure the
width of a pulse is to use the oscilloscope's automatic measurement.
For the oscilloscope to measure the pulse width automatically, the
time/division control must be set so that the entire pulse, including the
leading and trailing edges, is on the screen. Thus, the resolution of the
measurement is limited by the time base resolution. In digitizing scopes, the
time base resolution is primarily limited by memory depth and by the
intrinsic hardware resolution of the fine interpolator in the time base.
It is important to select the optimum time/division scale to achieve the best
resolution and accuracy. Both edges of the pulse must be on the screen, and
at the same time, there must be sufficient baseline (or top for a negative
pulse) for the top and base algorithm to reliably determine the voltage level
of the base (or top). Refer to the discussion on page 5 describing the
algorithm the oscilloscope uses to determine the top and base levels.
If the baseline is distorted prior to the initial transition of the pulse, enough
of the "true," steady-state baseline must be present on the screen so that it
will be identified clearly in a voltage histogram as the most prevalent value.
The minimum time/division setting that achieves this will give the most
accurate measurement. In other words, spread the pulse out to occupy as
much of the screen as possible while allowing the base (or top for a negative
pulse) to occupy at least 10 percent of the display (1 division).
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