Hope Todd doesn't mind me butting in here...
My experience is that you can "get" the absolute basics of floating thumb in a fairly short period of time. However, fine-tuning the technique is an on-going process.
I had been playing with a fixed-anchor type technique for over 30 years (with a several-year self-imposed layoff in there, though) and had a reasonably good handle on that technique.
I switched to floating thumb because I started developing wrist/carpal-tunnel issues in my dotage.
I "got" the floating-thumb basics fairly soon, thanks to Todd's most excellent "Technique Builders" DVD; but for a while I, too, had issues with my thumbnail.
Now, a couple of years later, I have no such issues with my thumbnail - and looking back, I can see that it has been "refine, refine, refine", paying attention and making small, subtle changes over time to my floating-thumb technique.
So, be patient, stick with it, etc. and you'll get there. The more time you spend with it, the more "aware" you will be of your right hand (esp. the thumb itself) when playing & you will find you can likely make very minute/subtle changes in hand and/or thumb position & angle as you play.
I guess I'm saying that, for me, it isn't a static "always-hold-your-hand/thumb-in-exactly-one-frozen-position" kind of thing, you know?
Just my own personal experience... hope it helps a bit.
