HI anil..
I was inquisitive when I saw your post..I tried out all sources but could not get what you want. To set a similar kind of alert, you need to calculate the size of the DB much earlier. Because the Default counter available will only recognise the calculated value and no % can be mentioned. Possibly you can go for a trigger which would activate once the spaceused goes beyond 70%. Please let me know what solution you adopt.