The only way to know how much trim weight is enough is to try it. Some divers need more weight on their back, some high up near their head, some need ankle weights. It varies greatly
Actually, I thought the matrix was a softpack harness like a Transpac and it would mount a single tank directly without using a backplate. Anyhow, on my Transpac I have used Halcyon trim weight pockets on the top tank band. Lately, I moved these trim pockets to the waist band to use as a limited capacity integrated weight system at 5 pounds per pouch maximum. Next time I go diving (Sunday, yay) I will try threading a pair of 2 pound weights on my top tank band, tucked down next to the back of the harness.
The Halcyon pockets are now $46. I was lucky and bought them last year for $21 before those so-and-so's realized they could squeeze more for that product. If you like pouches, Zeagle also makes some. I wonder if Deep Outdoors has some product that fits your needs, or if that might even be built into the matrix?