Excel.
In all seriousness, just a normal run of the mill accounting package like Quickbooks is fine - the only thing is you have to be careful about how you enter things due to tax implications, etc - and really there's not much magic to even that, you just need to be consistent. We maintain two sets of books - hey now, not in the illegal sense of the term, but meaning one rough and easy to modify on-the-go version and then our actual day to day accounting. For our on-the-go, we track things like ROI, hardware prices, exchange fees, pool fees, machines deployed, failure rates, etc. You could build something more robust, but in reality Excel does a pretty good job, just be thoughtful about it and you should be good to go. Then you can just compare your projections (the on-the-go books) vs actual, and that will give you the typical error you can expect - in our case, our quick and dirty tracking is about 10% off of actual, so it's super helpful from a forecasting perspective and making you aware of what you might need to keep an eye on.
Also make sure to do a monthly budget, and have your on-the-go books tick that down, that way you can see something going off the rails before you've burned a bunch of cash - or conversely spot something that's maybe doing better than you expected, and shift some resources over to it.
