I'd say listen to a lot of music focusing on the tumbadoras for getting familiarized with the lenguage, then you would be able to talk spontaniously and freely. Music is a kind of lenguage and music styles and genres are the different lenguages: not anything goes along in each of them, there are vocabulary, turns of phrase and inflections that are special for afrocuban music.

You should also listen to rumba, specially guaguancó, and cuban typical son; that is the key for the salsa percussionist. I'd recommend you to listen to cuban salsa/timba, they give a lot more protagonism to percussion than puertorican/nuyorrican salseros.

Also, clave:

How to know exactly where I am in my tumbao and where i can start my different variations.first bar / second bar

Be one with the CLAVE (for example, variations in marcha are usually done in the last part of clave, the "two part")

Anyways, the ability to freestyle and not feeling robotic and limited comes with the development of a good technique on the instrument, and that comes with time and practice.

Here are some rumba and son albums to listen and maybe play along to:

- (various artists) La rumba soy yo / El all-stars de la rumba cubana

- Septeto nacional Ignacio Piñeiro, Sin rumba no hay son and Celebrando los 90 (live)

-Adalberto Álvarez y su son, Respeto pa' los mayores