One of the very best tutorials I've seen on sewing the dreaded "Y" seams is the Kay Woods Hexagon Tutorial. Way faster than marking; Kay Woods has a set of established rules for setting in "Y" seams. Also, the "Little Foot" is a great foot to use when setting "Y" seams rather you are sewing Hexagons or Diamonds; just start and stop your diamond/hexagon piece at the marked front & back red lines. Kay Woods "Easy "Y" Seams" Tutorial:
https://www.youtube.com/watch?v=XLkHfcuNzCA