Here's a Reddit thread about AST stat testing in Diurnal: https://www.reddit.com/r/ffxiv/comments/54f2hu/whm_and_diurnal_ast_healing_formulas_and_more

As others have said, for raid healing, go for 592 Acc. Crit gives you the biggest healing and DPS boost, and Spellspeed is slightly better than Determination in longer fights (explained here).