Yeah I have a few 3 piece masts too

No Billy, you can't cover 4.0 - 8.0 with one mast.
I have 4.2 to 6.1 with two masts, and the 4.2 is not quite ideal on my 400, really should have a 370. So to be withing manufacurers spec, you will need about 4 masts, maybe get away with 3.
Get a pencil and try to bend it. You can't. Now go to the hardware and pick up a 2m length of dowelling the same diameter. Floppy as hell.
Thats why you can't have a mast with the same properties when it is extended lots, it simply won't work.