You need at least 6 feet plus. Refer the to "20 series unloader extensions" post from a couple weeks ago. It looks like 2- 3 foot extensions plus a Stewart Steel extender will you get you 3 feet past your header.
Maybe I'm missing something. His header is 5' wider or 2.5' on each side of the combine. Why would a 3' extension not give him enough assuming he has a 24' auger like we do on ours? If you have a 21' auger then yes you need 6'.
may be case wants to sell short augers so they hit trucks or bins, so then you would have to buy another header front. i had a look at our combine and on the auger, it has already the extra plates for where the auger sits back into the transport saddle. it looks like you can extend the auger another 12 feet from standard length, but case ih australia won't put anymore extension in. wolters
We have real life testing with a 24' auger and a 40' MacDon this will be our 3rd year now. We unload on the go 90% of the time it is tight, probably 3-4 feet between the head and grain cart, but we have never hit. Yes I would prefer more room but it works. So a 3' extension would work on a 45' head because you are adding 3' of auger but only 2.5' of head on that side. If you are going to the trouble of adding extensions I would add 6' without question just to reduce the stress of unloading on the go.