Just to dwell unnecessarily on the issue of timing the field's length.....one way would be to always fly the timing leg at the same speed and work out beforehand how many seconds you require. The effect of wind could be allowed for by flying slightly faster into a headwind or slower with a tailwind (or just adjusting the time using common sense.)
The correct formula is 1/2 ground speed x time (in sec)
If you're interested, the way the simple formula is derived shows how accurate it is:
If your GS is say 80kts, that's 80nm per hour. To time a field and get its length in metres we need to first convert 80kts into metres per second.
So 1nm = 1852m and 1hr = 3600 seconds. So 1nm/hour = 1852m/3600sec
1852/3600 is pretty close to half.... It's 0.514 in fact.
So to convert your speed from knots to metres per second, just halve it and you'll be within about 2.8% of the correct answer.
In the 80kt example, if you timed a 14 second field, the correct distance would be 576m. Using the rule of thumb, you get 560m. 2/5 of bugger all difference.
(Also useful to know if you ever fly in china, where things like wind are expressed as m/sec.)