You can't really, because the percentage is a non-weighted average of the fraction of achievements you have in all the games you have 1 or more achievement in, which means earning 1 more achievement in a game with 5000 of them will give you 1/5000/100/n percent increase, whereas if you got the second and final achievement in a game with 2 achievements that you had one in previously you would get a 1/2/100/n percent increase.

For example with two games if you had one at 10% and one at 22% you would have 16% on average (no matter how many achievements each has), and either reaching 12%/22% or 10%/24% or 11%/23% would get you to an average of 17% between the two. If the first game had 10 achievements you could sail all the way to 21% for a single achievement, or if you played the second game and it had 5000 and you got 1 more achievement it very likely wouldn't change your visible percentage due to rounding.

So it depends which game you play and how many achievements they have how fast the number will go up, but conversely it usually takes longer to get achievements in games with less of them than ones with thousands, so while numbers wise low achievement count games matter more, timewise it is dependent on how hard/time consuming the achievements are to get in each game combined with how many there are.

Originally posted by sfnhltb:

which means earning 1 more achievement in a game with 5000 of them will give you 1/5000/100/n percent increase

You have games with 5000 achievements ?!

Originally posted by sfnhltb:

the percentage is a non-weighted average of the fraction of achievements you have in all the games you have 1 or more achievement in

If I understand correctly it is better to complete achievements for games where I have no achievements and get first achievement for them...

I learned from a statistics teacher that averages are sensitive to extreme values, so it maybe best to complete achievements for games that have a percentage much lower than the total average of achievements on Steam or much higher, and it is less efficient to obtain achievements if the number for a game is close to the total average. For example if my total average is 17%, the best is to complete achievements between 0 and 12 or 13%, or 20 to 100%.

Do it manually.
Just make a note of the exact number of achievements you need to hit one more percent.
It will a good guide for what the number of achievements will be needed for the next percentage point.