In daylight and on relatively easy ground, the time to reach a target can be estimated reasonably accurately, and this can be a useful alternative in featureless areas.
At an average walking speed of 5 kph (kilometres per hour), it takes 1.2 minutes to cover 100 metres.
For each 10 metres of ascent, an extra minute should be added.
The time for each leg is pre-calculated and a stopwatch is used to measure the time taken (many watches and mobile phones have a stopwatch function).
Thus a leg of 300 metres with 35 metres of ascent will take 3 x 1.2 minutes (3.6 minutes), plus 3.5 minutes for the uphill work - i.e. 7.1 minutes at 5 kph.
Each 0.1 of a minute is 6 seconds, although its unlikely that this level of accuracy would be achieved.
Rough ground or darkness will interfere with this calculation, however.
The speed of the party can be measured by timing the group over a level section with a known distance - i.e. between two easily identifiable points with distance measured directly from the map.
At 4 kph 100 metres will take 1.5 minutes
At 3 kph 100 metres will take 2 minutes.
At 2 kph 100 metres will take 3 minutes.
A heavy backpack is likely to reduce speed by at least 1 kph.
Walking downhill is timed at the same rate as level walking although if the downhill slope is gentle it may increase speed by about 1 kph.
Timing is also affected by wind. A strong headwind will slow things down, and a strong wind at your back will speed things up.
You'll just have to practice in all sorts of conditions until you find timings that work for you.