I would put the boat in the water and let the water make a scum line to follow.
All boats sit in the water differently depending on engine size and equipment etc.
If the boat is in good shape I would seriously consider applying a barrier coat before bottom paint. It protects the gel coat from absorbing water and will help keep the value of your boat. Many might think this is over kill for a V20 but these hulls seem to be increasing in value esp if they are well cared for.
I've used water base bottom paint on all of our boats the past two season.
It works surprisingly well. Easy clean up and the paint goes much further then solvent base paints I've used.
This is the paint we use.
http://www.jamestowndistributors.com...+Bottom+Paint+
A gallon of Hydrocoat does our 35' Bertram.