Sorry to hear of your woes Peter. Supply chains, particularly those from Asia, are all over the place at present. It's an ongoing source of frustration when buying fishing tackle just now. Should get better with time though as the effects of Brexit/Covid19/stuck container ships get sorted out.
As to your question on braid measurement, I'm not sure I have an answer for you other than to say that when filling with 100% braid, this issue doesn't arise - I just fill the spool until the braid is just below the spool lip.
However, I appreciate the need for reasonable accuracy in your case to split the bulk spool evenly across six reels, so you might try getting hold of a line counter and rigging it up on the rod that holds the reel you're filling. This should give you a way to measure length of line/braid consistently.
https://www.amazon.co.uk/Berkley-Clip-on-Line-Counter-Black/dp/B005OTYW2E/ref=asc_df_B005OTYW2E/?tag=googshopuk-21&linkCode=df0&hvadid=275797872660&hvpos=&hvnetw=g&hvrand=13605067761123563019&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=1007009&hvtargid=pla-448867667493&psc=1