Attenuation for brewer's yeast generally ranges from 65-85%, and could be 60-90% if using lots of simple-sugar adjuncts or lots of unfermentables. That's a big range. For a "normal" beer, with an OG of 1.060, the range would be between 1.09-1.021, which is so large as to be practically useless.
To just pick 75% attenuation as your estimate is a sure way to be wrong almost all of the time.
If you have specs for the yeast strain you're using, it's a good start. For instance, the expected attenuation for WLP001 is 63-70%, so the beer in our example would finish between 1.018-1.022, which is a much more reasonable range, but is still a larger range than I feel comfortable trusting.