Probably not at 300mm, but possibly at 70mm.
Extension tubes have maximum effect on focus distance with short focal length lenses. It is possible to calculate the approximate
change in magnification and focus distance, but it's a rather complex process. I should probably write an article on how to calculate it, or at least how to get some sort of rough approximation of it. For a simple one element lens it would be easy. For a complex zoom lens it isn't because it depends on knowing the effective focal length and positions of the principle points
Focal length isn't as easy as reading it off the lens scale because almost all zoom lenses actually change focal length when close focused, so a 70-300 set to 300mm and close focused probably isn't acting as a 300mm lens anymore. It might be as short as 200mm.
If you just look at data for some older 100-300 zooms (see http://www.bobatkins.com/photography/eosfaq/closeup.htm
) you can see that the minimum focus distance with a 25mm extension tube was around 1.2m at the 300mm setting and 0.6m at the 70mm settings (focus distance is defined as the distnce between the sensor in the camera and the subject). Without the tube the minimum focus distance of those lenses (see http://www.bobatkins.com/photography/eflenses/lens_list.php
) was around 1.5m.