50lb grinder braid is not 0.28mm, despite what the box says.
It is physically impossible for a 50lb braid to have a dia of 0.20mm, unless it is made from unobtanium.
Typical actual breaking strain for a PE line that really is 0.20mm in dia will be around 15lb. Really good stuff maybe 18lb
Buy a 600m spool of 50lb 8 strand HMP and fill it, you will have about 40m left over.
If you must split it backing + topshot, 300m 50lb 4 strand HMP + 300m 50lb 8 strand HMP. Done.