lol, i can see it now, Nagg walking his line out while kids are getting coat hangered by fishing line as he tries to wind it back on....hahaha. Me, well I have a big enough yard to do it at home and coat hanger the dog instead!
I don't think there's an easy way around this to get the whole sppol of braid on without falling short or over spooling. Try it with the amount of backing you desire, then put the braid on, if too much line run it out and cut off the desired length of mono (from the spool so you don't have to retie the braid on) and keep going, trial and error until you are happy with it. You should only need around 10m of 30lb mono to back the braid, maybe slightly more or less depending on diameter
I have an old reel that I use specifically for loading spools. Works for me because I like to use the reel drag to get the right load/tension.
I use a metre or so of leader to tie on to the spool and then full braid.
put on your desired amount of braid.
add your mono to fill the spool.
take the mono off and keep on a separate line container.
take your braid off and keep separate.
put your measured amount of mono on the empty spool.
and top with yor braid.
saves spooling 3 times
I've been trying some Daiwa PE Braid recently, and I wouldn't recommend it for Barra. I've been casting Spinnerbaits on 15 and 30 both off Zillions and the Branzino. I don't think it's a tough enough line. I tried 45 on the Black Sheep for barra and I put 60lb on the Hyper 4500 for PNG. The 60 is the lowest I'd go for real Barra fishing, it is about the same strenghth as 30lb Bionic.