Adjusting OD shifting

Any good theories or rules for adjusting the OD shift parameters?

My truck now has a turbo, and apparently it's highway manners are
changed enough that it's kind of odd when it comes to "passing" on
the freeway. Too little gas and apparently it's not enough juice to
actually speed up, a mild amount of gas and it'll downshift to 3rd, we
get some acceleration but it shifts back up to OD too quickly, and if
I give it a good amount of gas, well, it's now a turbo so I'm going to
go way faster than necessary. :)

Basically it seems like I need for it to shift down to 3rd easier, and
stay in 3rd longer. Are there any rules of thumb to follow when I'm
working on the tranny tuning? I know that's a broad question.