Total revenue divided by total marketing spend across every channel. MER (sometimes called blended ROAS) is the honest counterpart to platform ROAS: it cannot be inflated by attribution games, because it only uses two numbers you cannot argue with, real revenue and real spend. If platform ROAS is climbing but MER is flat, the platforms are claiming credit for sales you would have made anyway.
Benchmark. MER targets are stage-banded: roughly 1.5-2.5x at $1-5M revenue, 2.5-3.5x at $5-10M, 3.0-4.5x at $10-25M, and 3.5-6.0x+ past $25M. As a rule of thumb, 3x+ once past ~$10M; 4x+ is strong. Your specific target also depends on gross margin.