One option, though you may find it graphically shocking at first, could be to use a right tab instead of a left tab: then the space between the numbering and the text will stay the same.

Another is to stay with a left tab but deliberately use a wider gap. If all your body text is indented, for example, you can leave the numbering in the left margin (which makes it easy to scan) and align the text heading with the indented body text.

A third option, of course, is to consider whether you really need numbering at all! Cross-references (even if it does seem to take two xrefs in Word to get "See title on page") and clearly differentiated styles use the power of your word-processor to make structure visible and numbering redundant. They can also, I believe, be more user-friendly: consider content where 10.2 is one page away from 10.1, but four pages away from 10.3 … I've just checked the four reference books next to my PC, and none of them uses numbered headings.