Yes, that's exactly what you should do.

When you mix fertilizers to the value you set or are targetting, you must adjust for all the non-fertilizer ingredients. One way to do this is to add your supliments to the reservoir water prior to adding fertilizers. Take the TDS reading from that mixture and add that to your target fertilizer TDS.

Using your example.....Your ending PPM should be your target fertilizer PPM (1000ppm)+ beginning tap water (80ppm) + any supliments ppm.

Something to remember. Don't max out your fertilizer PPM and assume any additional TDS inside your reservoir is harmless. You have a maximum TDS (of all ingredients) that must not be exceeded.