Do a simple calculation of the percentage of total votes cast by party and compare to the percentage of seats won in the House. Republicans currently hold a +2 edge compared to what a proportionally representative D-R split would be. In the last Congress it was a single seat and both those small margins can largely be explained by single district states that have fewer voters in their district. California pushes seats to the Dems, Texas to the Republicans, etc. and it more or less washes out in the end on a national level.