The information I have received, by some credible “Team Boaters”, has consistently been this.
Place your boat on a flat hard surface then slide a broom handle, or a wooden dowel, underneath your boat. Then while sitting in your boat, there are 3 main sitting positions, Aggressive (leaning slightly forward), Neutral (sitting straight up) and Defensive (leaning back). The boat is properly ‘trimmed’ if you’re in the Aggressive position (slightly leaning forward) and your boat is perfectly balanced on the dowel, front to back, and the dowel is directly below your belly button. Then if you lean further forward your boat tips forward on the dowel and if you sit straight up your boat tips back on the dowel. This can be difficult to perfectly achieve but having an extra set of hands to help balance makes things easier. The final test would be when forward paddling in flat water, while in the Aggressive position (leaning slightly forward) your boat should be balanced front to back, then if you lean further forward the nose of your boat should dip down slightly and if you sit straight up you should see the nose of your boat pull up. The goal is while in the Aggressive position (leaning slightly forward) and this should be your normal paddling position, the boat should be balanced "trimmed" front to back on an axis directly below your belly button.
Hope this helps and IF your wife walks in while your sitting in your boat while on top of the kitchen table and a broom handle sticking out from underneath, don't take it personal if she rolls her eyes and walks out of the room, I promise you'll laugh together about it soon.