Ron,
It's important to always sing at a comfortable level and never, never strain to hit the high notes. A good rule of thumb is to find the highest note in the song and transpose until you can hit that note comfortably. If you have to strain to hit the high notes it can result serious health issues such as vocal cord nodules and blisters, both of which can become cancerous.
Cheers,
Gary
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Travlin' Easy
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K+E=W (Knowledge Plus Experience = Wisdom.)