Whenever you present a piece of analysis (a table, a chart), round up the numbers so you are left with a digestible amount of numbers; so $1.3m instead of $1,354,673. And when you add up numbers, make sure the calculations are correct. And I do not mean just calculation mistakes, it is obvious that you lose credibility if you get the basic math wrong.
Adding rounded numbers can slightly alter the total of your sum. I usually make sure that the total is exactly what it should be, and make a short adjustment to the largest number in the addition.
For example: 3.49 + 2.55 + 1.25 = 7.27. But when you round up you get: 3.5 + 2.6 + 1.3 = 7.4. So I adjusted the 3.5 and will put in my chart 3.4 + 2.6 1.3 = 7.3.
Why? If you have to explain why numbers do not add up, it will cost you credibility. Secondly, most people actually will not ask, they will just go and check every number in your document. And an audience that is running mathematical calculations does not have time to listen to your story.
Watch out the quarterly results presentations to investors though. If you bump into rounding issues, you might have to add in those extra digits to make sure you are not misrepresenting the financial situation of your company,