TalkChess.com

[Mincho Georgiev]

It's much more complicated than just x% of the time, because you will have a very high number of unfinished iterations that way.
What I do (and probably others) is to use time boundaries. One for the absolute maximum allowed (which can be considered for the X%
that you mention, just bigger) and one for the regular slice. If you accomplish the "regular" boundary but the iteration isn't finished yet,
you keep going until the max boundary is reached. On the other hand, if you have let say 70% of the regular finished, don't go for new iteration,
unless something bad is happening (or whatever you decide) - if you do that, use the max boundary.
You have a wide variety with this method and you can choose what part of the max to use and when and so on.

Best Regards!