Yes it is better to say "he can be sentence to no more than 20 years if found guilty". The 20 years is just a limitation on the court's discretion: hindering a federal investigation is never so bad that a person should be sentenced to life in prison or death or a 32 year term. But it might bad enough that 16 weeks or 30 months or 19 years is appropriate depending on specific facts.
And when the court does sentence a person for a certain offense, it should compare the specific facts of the case to the worst possible case, the one that would warrant 20 years, and if this is somewhat less than the worst possible case, then to sentence them to an appropriately shorter term.