There is a book right there!
Turbine tailpipe design has the biggest influence, pipe diameter and design (bellmouth design) Small lip flared tailpipes, should have a smaller gap.
Best advice is from your turbine manufacturer. Like you say 25mm is a good starting point on single cone byepass system installs. The old P-80 type with twin cones needs a larger gap measured to the outer 'short' tailcone-around 50mm.
Many just install the turbine where it sits. As turbines have got longer, its more difficult to get the turbine far enough forward.

Models like the Flash/UF that have been around since 2006 need the 3mm front turbine rail vertical former cutting flush to the rails so a JetCat turbine can be moved far enough forward.