On one of my early profession SM gigs, I asked the TD if he'd mind assisting me in taping out the set. I was honest with him and let him know that I didn't have a whole lot of experience in that area.
I once asked in college and the TD, Designer, my staff supervisor all decided to not help me. It was an amazingly fun time. And the floor was never taped right....ever. This forced me to take an intro to tech prod class when I was a super-senior. I gots to hang out with the freshmen, but I finally learned how to read and scale a ground plan.
But to the topic at hand: I'd assume you've already been told you are allowed to tape wherever you are rehearsing. There are many rehearsal studios you'll encounter that forbid you to tape out anything because of the amount of traffic in and out of studio spaces. It may be a moot point if you can't tape at all.
While precision and accuracy are always nice, don't be too concerned if your set doesn't fit perfectly in the space. I've found in general, you can scale your set down about 3-4 feet without anyone being able to tell the difference when they are in the space with the actual set. Don't hide the fact that you've done it, but if you're a few feet off and you shrink things, no one will be able to tell the difference...having wings tends to make people think they have more space.
Beyond that: Maribeth nailed it.