Steam Summer sale tools

Steam Enhanced addon

Yes I have that thanks and its very useful to tell you how cheap game has been in the past and where it is the cheapest. What I wanted was a site where you search the game. i.e. Legend of Grimrock 2 and it says what the discount is now i.e. 50% but it
tells you that it is likely to go to 75%.

Just wait to see if it gets to be a daily or flash deal, if it's not, buy it on the last day

edit: also, check if the game has seen a better sale in the past on steam. if it has, there's a reasonable chance it will hit that low price again

There's nothing to tell you if it will likely go on a bigger discount because that's basically impossible. It would just be guessing. The only thing I can recommend other than Steam enhanced addon is the site isthereanydeal.com. Pretty useful for notifying you if something goes on sale in the future, and you can put your preferred discount percentages and more.

http://steamdb.info/sales/ will tell you if a game currently on sale has had a bigger discount in the past.

Also, you can search for specific games here: http://steamdb.info/search/ and it will show the current price and lowest recorded price for several different currencies.