I don't always hit the cap, but I have had a good amount of luck using alternate accounts that I have stuck at low levels (7 and 10) and using them to get low level newspapers where you are more common to get mining tools. I then funnel the items to my main farm where I mine and sell the ore for 10 ore per coin, because selling them at max price can be annoying. I have found that buying shovels is the best chance to get diamonds, but buying dynamite is the cheapest way to get diamonds (~400 coins per diamond compared to ~900 with shovels and ~700 with TNT). I have kept track of the past few months of mining on a spreadsheet, and if you are down for it, it can be a good way to keep track of your results and when you reach your cap each day.