I play lot of game with my 15mm miniatures instead of 25mm. At each time I use 1,5cm (15mm) for one inch in 25mm scale.
it work well.
I thinks only 1cm instead of one inch is too short and it's too dificulte to touch and move the miniatures.
1,5 by inch is a very good compromise.
For exemple, a 25mm table 36"x36" (90cmx90cm) will be a 15mm table 54cmx54cm.
With this exemple, you could see that, if you take 1cm for 1", a game table 36cmx36cm is very too little. No place for your big fingers
It's easy to remerber, 1" (2,54cm) for 25mm minature and 1,5cm (15mm) for 15mm miniatures.
But if you've got some problems with metric system, I belive you coud choose 1/2" for one in 15mm scale. It should work weel too.
Stéphane