Credibility grows when your claims routinely match what others can verify.

Credibility is really a track record of your statements surviving contact with reality. Every time what you said matches what someone later discovered, the account gains a little. Every mismatch, even a small overstatement, takes more out than the match put in. People are unconsciously running this tally on everyone they know.

Get in the habit of understating rather than rounding up. Give the number you can defend rather than the number that sounds better. Over a few years the difference in how your word is received is considerable.