Track achievement progress.
Achievement completion is unlocked count over total count, but the more useful figure is your unlock rate — achievements per hour played. Projecting the remaining achievements at that rate gives an honest finish estimate, and it is usually pessimistic in a good way: early achievements come from simply progressing the story, while the last few are typically grind or skill gated and take far longer than the average suggests.
Completion percentage
Completion = unlocked achievements / total achievements x 100
Projected finish
Unlock rate = unlocked / hours played; hours to finish = remaining / unlock rate
Because achievement difficulty is front-loaded in your favour. Story and tutorial achievements unlock quickly, so your early rate is inflated compared with the collectible and mastery achievements left at the end.
Only if you intend to earn them. Most platforms report a base-game percentage separately, so mixing DLC into the total makes your figure diverge from the one shown on your profile.