You've got the right idea. Parallel parking just takes practice. Another tough issue it provides is that it works differently with power steering cars. Practice with a car that does not have power steering in order to prepare yourself for the cars that testers use. My parents had an '89 Jeep Cherokee that was a breeze to parallel park, so I got a nasty surprise when I over-confidently tried to parallel park in driver's ed.
Maybe they'll let you drive your own car during testing. As a native, I took driver's ed at 15 and passed with good marks (since rural driving gave me plenty of underage practice
), so I never took that assessment drive (or whatever it's called) at the DMV.
Since you probably haven't needed driver's ed at your age (I forget the laws, but I think 18+ could just do the test), you should know a fairly dependable general rule they teach you. If you mess up while following this rule, an instructor will understand that you know what you're doing but may just need a little patience for adjustment to the different car.
First and foremost, you want a good side-to-side proximity with the front car when you initially line up. This will make or break your park, and unfortunately is only attainable from practice specific to whatever car your driving and how that car steers. There is a generally reasonable distance, though, but I've never heard a good standard rule for judging that distance. If someone else could provide that, I'd share in your curiosity.
Next, as an aside, your right door mirror is one of your best friends. Use it properly and you'll see why the average driver's "guesswork" isn't a problem for truckers (and you'll wish you had one of
their mirrors
). Objects may be "closer than they appear", but whether or not they're about to touch side-to-side is clear and accurate as can be. This is very helpful in the initial part of the backup, as well as when lining your back tire to the curb in an extra long or short car (that's only if you can see the curb easily in the mirror... the person testing you won't appreciate a mirror adjustment or strained pause mid-park, even though that would be ideal for the curb). In any case, you can use the right side of the bumper of the front car to gauge the point at which your back tire should be straightened out.
Now, as you're backing up, you want to crank the wheel for a hard right
until backing up puts your right mirror in line with the front car's back bumper (or cone). This is the point where you crank the wheel to your left and
briefly watch your front right corner to ensure it's easing in properly (for this reason, of course, the mirror line-up rule has to be intuitively modified for longer cars or when you're too far/close with side of the front car). Finally, once you straighten out, it goes without saying that you want to not hit the back car/cone.
Ultimately, if the instructor sees you're judging initial side-to-side proximity reasonably (not too close or too far in any obvious way), focusing on the necessary potential points of impact, and
following the general mirror line-up rule, you'll get the benefit of patience for any unusual aspects of your testing car's build or steering.
Sorry if I sound like a know-it-all. Parallel parking is very intimidating to the inexperienced driver, but once you do it and think through each different moment, and even more so once you've driven a large truck or with a trailer, the whole thing just turns into knowing where to keep your attention and when to change that focus, and from there experience just leads it to become second nature. Really, you can't see the front corners, grill, and tires of your car, but you can guess when they're nearing impact. In reverse, you even have your side mirrors on your side (yay... a pun!). Reverse is just more intimidating due to instincts and lack of practice.
And once again on that note, you've got the right idea with practicing.
Best of hopes.
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Drivers test
) and people who go slow in the left lane, driving with almost intentional perfection to go the exact same speed as the cars to the right of them. Next thing I know, I've got a row of a dozen tailgaters behind me, waiting like a sequence of dominos next to a time bomb.
And I won't even mention California, although one thing they do have going for them is an Eastern-style "fast lane". 
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