NEW YORK — The slight sales boost that Memorial Day weekend brought to Broadway mostly evaporated the week of May 24-30, with Rialto cume off about $850,000.Overall slippage was likely offset somewhat by the fact that the Broadway League, which last week consolidated the reporting of Main Stem numbers, began reporting “gross gross” vs. the traditional net gross (which subtracts credit card transaction fees from totals). Down about 4%, Week 1’s tally hit $19.6 million for 32 shows on the boards — while over the same frame in 2008, sales fell more than $3 million (15%) to $17.5 million for 34 productions playing. “Joe Turner’s Come and Gone” ($283,197) — the most talked-about offering of the weekend, thanks to a highly publicized May 30 visit by President Obama and wife Michelle — didn’t climb by a huge amount, but the coming weeks will indicate whether the recent publicity will lead to a sustained increase in sales at the Lincoln Center Theater production. The 20 musicals grossed $14,702,527 for 75.2% of the Broadway total, with an attendance of 186,006 at 88.7% capacity and average paid admission of $79.04. The 13 plays grossed $4,838,507 for 24.8% of the Broadway total, with an attendance of 74,680 at 58.6% capacity and average paid admission of $64.79. Average paid admission was $74.96 for all shows.