In reading the IRS instructions for the extension to file, Form 4868, I cannot discern some facts about the costs of not paying the full tax owed with that form, by the April 15th deadline. 

With regards to interest on the unpaid balance, it does not specify the interest rate. Is that rate known or predictable?

For the penalty on the unpaid balance, it says you will be charged 1/2 of 1% for each month or part of a month that the tax is unpaid by the regular April 15th due date. Then it says: "The maximum penalty is 25%." What do they mean by the maximum penalty is 25%? Is that the maximum monthly penalty rate, or the maximum total penalty owed? (Thus, if it was the latter, and you owed $100, the penalty would not exceed $25, regardless of the number of months.)

It is clear that the interest cost continues till the balance is paid.

Thank you very much!