The spec is in the owners manual. I don't have it at the moment, but believe its 17 or 19 mm. Honestly, between those two numbers and the carb runs nicely, IMHO. What's the problem?
I've sure never found it in the owner's manual that I have.
It's not in Pete Snidal's Service Manual either. He goes through the BS-29 (except for a good exploded view) pretty well, but never mentions a float level height.
I've never found it anywhere & if you want good tuning info on BS-29s you've gotta Google Suzuki or another that used these carbs back in the '80s. And read all other Mikuni BS CV sites you can find online. I've never found ANYthing on BS-29 specific to Royal Enfield except some useless mixture screw info mentioned in the Owner's Manual. Maybe the Factory Manual has it, but I don't have one.
There's also a lot of BAD info on BS carbs in a lot of the online forums. Mostly involving the mixture screw & which way is rich, which way is lean.
Aquaholic - Why do you believe your float is a problem?