Well, not exactly. The D+ number is always based on turnout at the LAST election.
Whatever is “the” accurate number for D/R registration, it is somewhat meaningless because 50% of a much bigger registration number can lose to 95% of a smaller number.
The reputable pollsters will allow D/R/I to fluctuate in each cycle. Those that model it on the last election are usually wrong. The electorate obviously shifts a bit from cycle to cycle. The last 3 elections were Even (Bush 2004), D+7 (Obama 2008), D+6 (Obama 2012).