A four foot by four foot table has 2,304 square inches.
For gaming terrain to cover 25% of the table, that would mean you would need 576 square inches of terrain.
If your terrain pieces are six inches square (and most of mine are, because I’m curiously compulsive that way), you would need sixteen pieces of terrain to make up 25% of the table.
For ruined fieldstone terrain, I have four, and a fifth on the worktable.
I have dreams of being able to one day field a gaming table for a skirmish game, one that would have serious terrain to play around. Doing simple math like this makes me realize how much really needs to go into something like this. I’ve seen some tables for Mordheim (and similar games) that have just under-decorated, and tables like that heavily favor missile weapons as a result. I want a table where Chaos Cultists and Sisters of Sigmar have a chance against Reiklanders or the like.
Anyways, I’ll hopefully have piece number five up by the end of the week. Stay tuned.