Replacement tweeters for tannoy p10

You need a ton of info to get right,  if you can get it right...if there is any pad in the original crossover that can effect the bsc.... The only Audax spec that I have is on a TW025A0... not the 01 version. Do you have a schematic from the Tannoy?  It isn't likely that the Audax is a direct drop in, you'll probably need to change a piece or two in the tweeter circuit besides BSC.
I assume that you measured DC on the tweeter and the width of your cabinet to figure BSC? 
If I can get enough info, I'll be glad to help,  Tim

Did a bit of reading on the audax tw025a0, found that the tweeter has a 6db drop after 4000hz. Think ill go with custom crossovers, not sure if i should cross them at 2200hz, or 4000hz?
  Current tannoy p20 are at 3000hz. I'am using loudspeaker design 2 on the link.
http:// https://drive.google.com/a/nezil.com/file/d/0B9nv0o7yxOlcaHczMllyQTRBdDg/edit?usp=sharing

The cabs are birch plywood 22L, 9" x 18"
pce

Well, the pdf I saw showed about a 4db drop at 7K.... Go here and confirm which tweeter that you have:
Go to the old Zalytron site.  www.zalytron.com
Left side, click on audax, scan to the bottom of the page, click on pictures & specs.... on the next page,  you see a French word for Archives...go into there. Find your tweeter and let me know.  Tim

after I get your correct schematic and the correct tweeter, you can pm me and I'll draw up new crossovers for you trying to keep your changes as minimum as possible,  but you will need some changes.