I haven't done a plate recently but back some years ago, I bought an aluminum boat that the capacity plate was missing from. I was in PA at the time. The boat registration form had a section you filled out with the dimensions of the boat and they figured out the capacity and made you a plate based on those calculations.
I have read various things on the issue of capacity plates. Federal Law requires a capacity plate on all boats less than 20' LOA. NMMA requires them on any boat under 26' LOA. You don't need to comply with NMMA as you are not trying to get NMMA certification.
The simple rule of thumb method is as follows:
Number of Passengers= Length X Width / 15
Or 7 passengers = 18' (LOA) X 6' (Beam) divided by 15. The number of passengers here is based on an average weight of 150lb. per passenger. Newer USCG capacity plates are based on 165lb. per person. Persons are live load which can effect stability. Total capacity, persons motor and gear for this particular size boat would be 1400lb vs. just 1050lbs for just persons.
There are more accurate and complex formulas for calculating it.
Here is a link to the full USCG document covering how to calculate it under subpart C:http://uscgboating.org/regulations/boatbuilders-handbook.php
I might recommend this document be made a sticky on the website if it isn't already.
Even if your not required to have acapacity plate, it doesn't hurt to know what your boats safe load capacity is just for personal satisfaction.