It's not how many times mate. Mesure once, then do it it once more just to be sure and then drill. As long as you concentrate on the job to avoid mistake you will be totally fine. Also, give the area you drilled a coat of paint to avoid rust on the exposed steel
Measure twice and do it well. The second one should be just to make sure the first one is correct. As the comment above says, give the naked metal some paint because if you don't it will rust.
