angle "°"/2;tan*1.5*Ø,
15°;tan=0,26794919243112270647255365849413*1.5=0,40192378864668405970883048774119*8"=3,2153903091734724776706439019295
dont let the long nubers scare you, they were copied and pasted.
about 82mm on a 8"*30° long radius elbow, or a little over 3 and3/16"