I would avoid matrix for B&W with filters. Instead, I'd spot meter the shadows, and compensate -2 stops from the meter reading. Use the camera's spot meter.
This is simple to do in camera. Set -2 stops exposure compensation on the camera. Set to spot metering mode. Meter the shadows and exposure lock. Recompose and shoot.
Matrix is for color. Works exceptionally well especially for slide film. It's not for filtered B&W, but that doesn't take anything away from it. None of the sophisticated modern scene evaluative metering systems are tuned to filtered B&W. And if you are shooting with a red filter, it is almost certain you are shooting landscapes and taking your time. Red is not generally a street shooter's filter, for example.