458 Coolant Flush

I'm replacing the coolant tank and doing a coolant flush on my 458. I'm trying to drain the coolant and have only been able to get about 2.5 gallons of out of it. I drained it from the front two rubber hoses that are right by the radiators, and also from the ones in the back right in front of the engine. I'd like to make sure I get all the coolant out since I'm not sure what was in there and don't want to have to worry that I'm mixing in compatible types if the previous shop put something different in there. I looked at the sticky and some of the previous threads on coolant flushes, but didn't see any more info. Is there any trick to get the rest of the coolant out?

 

Thanks flash, this is what I ended up doing. Was easy enough to do, but it took a few days with having to let it cool down after each run. Also now I have like 12 gallons of coolant to get rid of.
The last time I measured how much I got out so I could figure out how much water was left in the system and use the appropriate amount of concentrate. Doesn’t really have to be that accurate anyway as the car will likely never see temps below freezing anyway.
If anyone else soing this I found it easier to drain from the front (as per the workshop manual) than the back. I just shoved a plastic trim removal tool inbetween the hose and the aluminum tube to let it drain rather than fight with removing the hose each time.