The first thing that you should do if you want to get an online tax calculator is that it should be free. Unless you are a huge company with complex income structures, you should not pay for an online calculator. And even if you are a huge company, you will need to hire competent tax professionals to whom you can delegate the job of handling your taxes. The good news is that there are so many online calculators that are free and which do very clean jobs. You don't need to pay for something that you can get for free.
The second thing that you need to know is the issue of accuracy. This is especially true when you have a very large stake, in that, messing around with figures can cause you to lose out big time. When you want to test the accuracy of a calculator, try out with known figures first. Once the calculator gives you the answer that you know it should give you, you can then use it to help you on the areas that you have no idea about.
Lastly, you should always go for calculators that will not increase your chances of being attacked by an online virus. Avoid calculators with .exe extensions that will require you to download them. Use those which are based online or those with extensions that are for spreadsheets.
Once you are able to handle these three, then you will definitely not be in any problem because you will have found an excellent calculator that is affordable and safe.