What do you do when a sentence ends with a decimal?

Well, you have two options:

• Option 1: re-arrange your sentence or invent some spurious phrase to put after the decimal so that it looks less obvious;
• Option 2: don't worry about it and just have a cup of tea.

Either rewrite the sentence or paragraph so it doesn't end with the number, or write it as you should with any normal sentence and put a period at the end:

The answer is 0.8.

That looks much neater to me than The answer is 0.8 ..

If you're not dealing with mathematics or plain numbers, then another approach is to mention the units (which is often a good idea anyway):

The answer is 0.8 litres.

When we use a number without a decimal point at the end of a sentence, we use a period in a way we use it after the last word. The same rule should be applied for ending the sentence whether it ends with a word or a number, with or without a decimal. If you refer to some accounting books, they use it the same way you have used in your example.

Enter the total Taxable Amount of 400.00 and Total Tax Due City or Co. of $11.00.