You can use this calculator to play around...
I got close enough on a 1.3x crop just adding all three kenko extension tubes behind my Canon 100L macro. According to the calculator, that gave me a magnification of 1.45 (and then multiply by 1.25 to give 1.8).
That's at .25m. Keep in mind the lens plus body thickness to sensor is about .165m. So you really only have .085m working distance, or about 3.3". With the 500D (the 250D won't be able to focus), you'll have about 1.7" working distance with a magnification of 2.2. At f/16, you'll have DOF of 1/10th of a millimeter.
I captured the monarch egg with the extension tubes. But, I had a relaxing working distance of 3.3" and an enormous DOF of about 3/10ths of mm. :)
Monarch eggs are about 1.2mm tall, for reference, and these have been cropped slighty.
If you're going to do more than 1x life size, the Canon MP-E 65mm is what you want. A friend has that lens and creates remarkable macros.