RD28ETi Belt Lengths

[jp4294]

Hey guys,
Sorry if this has been covered somewhere but I couldn't find anything regarding specific belt lengths/codes.
I've got a '99 2.8 GU and I'm wanting to replace the AC belt but have no idea what length to get. Different manufacturers say different things and the Bosch one I have on there is a 13A0925 (the Bosch recommended one) but it's loose and the idler pulley is as "tight" as it will go (doesn't look like a really old belt either.)
Does anyone have any recommendations on a belt length they've used that works well? Maybe my belt has just stretched heaps even though it looks ok in appearance?