The best advice I can give Is try not to worry too much about being dead on, one thing that helped me is ESTIMATE where you think it needs to go, keep in mind you are able modify the model later.
I have come a long way since posting this and the issue I had to fix, was me not worrying too much at the start of modeling, to follow a guide to the exact measurement.
In the end, exact measurements ain't the best way to do things when you can most of the time, go back and touch up on the things you feel, need it.
That being said, to answer your question.
If you have already created geometry and are trying to fix your grid lines to match up with the lines on the Model, I suggest try scaling the model as well as changing the grid lines.
Hope this helps.
