A speech contest can spring surprises on the contestant. For example a whole chunk of material might evaporate from your memory (I should know that), you might draw a blank, the audience might laugh longer than you anticipated (if you did at all) or your points might rearrange themselves.
Therefore use your established word rate to determine how many words you require in order to end your speech with a minute to spare. For example if the contest requires a five to seven minute speech, such that your talk should not exceed seven minutes thirty seconds, try to end at six minutes thirty seconds. This way you have a buffer of one minute.