The current value, ( it is constantly being updated as new data comes in) as listed in the 2008 RASC Observer's Handbook is:
H = 71 (+/- 4) km/(s x Mpc)
The empirical relation appears to be linear over the distances and space we have good data for.
example: a galaxy located 10 million parsecs away (about 32.6 million light years) should be receding from us due to the expansion of the Universe at a velocity of roughly 710 km/sec.
Using this relation, the highest possible velocity of recession, c, occurs at 4.225 Gpc, or 13.775 Glyr. That's how they calculate the age of the Universe to be roughly 13.7 billion years.