Current BIS for Bard

[Bardo]

Hmmm, can't you then work out an optimal dex/det/crit balance (ilvl90 gear that you shuffle around for those stats) this way? If you do, then you could work out a bis to attain those stats ~.

You could figure out the item budget for secondary stats in each slot and then calculate an optimal crit/det ratio for i90 gear specifically if you could actually just have crit/det on every piece...
But since we only have 2 choices for i90 in most slots (3 in a few accessory slots) and nearly 80% of those pieces have either acc or ss on them, you can't come anywhere close to that "ideal" ratio.
And you can't simply choose the set closest to that ratio because you can't ignore Accuracy (for Coil set) and Skillspeed.

There are 27,648 possible combinations of i90 Bard items with Allagan Bow including the 4 main HQ food choices we have.
I suppose you could spreadsheet the combinations, sort them by accuracy to weed out anything below 472 and choose the set with the closest crit/det ratio to the ideal, but that would still ignore Skillspeed and River of Blood.
Better to use simulation software for this kind of thing (which has already been worked on by several members here).

There are several competing BiS Coil sets floating around:
Hoplite - http://ffxiv.ariyala.com/LG58
NightWrath - http://ffxiv.ariyala.com/LG48
BrandoH - http://ffxiv.ariyala.com/LMKI

I haven't seen anyone mention a BiS list for outside coil, but I'd imagine you just stick with Allagan bow and then try to ignore as much ACC as you possibly can since it is an entirely wasted stat:
Bardo - http://ffxiv.ariyala.com/LSX0