In a two column layout you only need to float the first div and then the following element will fill the available space automatically. The problem is that it will also wrap around the float so you simply give the second div a margin-left equal to the width of the float and then you have your two columns. The second column needs no widths and will fill the available space.
However, then we run into the question of what happens when you add clear:both to an element in the second column as it will also clear the first column. This leads us to your question about clearing floats and only floats in the same "block formatting context" are affected when you clear a float.
This page explains the problem and provides the solution.
Float clearing and float containing are different concepts and the clearfix method is a method of containing floats within a parent so that the parent extends around the float. Clearing a float is simply the matter of making content start after a previous floated element in the same block formatting context.
In your page you have floated two colums. One of 15% and one of 75%. That is a fragile way to build a page because as the screen gets smaller your 15% is unlikely to be able to contain the content it holds and content would overlap. You could set a min-width for the 15% column using px but then that would cause your right floated column to drop down when that limit was reached. This would not happen if you used the approach I mentioned above as the second column is not floated and needs no width applied.
These days if only IE8+ support is required you can use display:table and display:table-cell for columns quite nicely but it does then limit you to table concepts and makes it awkward if indeed you do want columns to wrap underneath or wrap around.
One thing for sure, I do have to do a lot of math to figure padding / margin. One trick I got from anothher humane society
site is using an empty div to push over other divs. Frankly this seems better than figuring padding or margins.
That sounds like a bad idea and is not needed.
I may wind up sticking with tables, big deal if the cells have content and not just data.
There is no need in this day and age to revert to tables for anything but tabular data. IE8+ all support the display:table and display:table-cell properties so you can mimic your beloved tables but using divs instead and not lose out on semantics as css implies no semantics on the html.
And real page wont have inline styles, but a stylesheet. I test a lot using inline styles.
Sometimes you may think its easier to test with inline styles but I can guarantee you that it us never easier especially when you need to make multiple changes. Don't worry, though we've all been there and done it ourselves and found out the hard way There's a lot to take in at first and you are never going to get it right first time so don't worry if you make mistakes because that's how you improve.