[PORKY]

I used a similar product made by SEM, but it wasn't self leveling. I didn't just seal the seam in the bottom, I filled the drip rail about half full and shaped it. It was a long way around that rail. I used 1-1/2 tubes. That stuff sets up quick too. If you are just sealing the seam, it should be much easier.
This is what I did.
1. Masked off everything but the inside of the rail (makes clean up easier).

2. I had a helper, to hand me stuff.

3. I was ready to smooth it out some before it dried. I took a 1/4", round, ratchet handle, that was the perfect width to fit the rail, and dipped it in surface cleaner.

4. I still had to sand out other inperfections when it dried too.

Maybe use a little of it first in a test piece. The similar SEM product I used was good stuff, dries quickly and does not shrink.

Good luck!
Frank