Well now, you are getting into my area of expertize..... the dollar 98 Wal-Mart calculator!! Lou missed a button his abbacus..... 13.3 mm is actually 0.5236". As to his comment about 0.007" being a reasonable tolerance, I would say it depends. In this particular case the proposed substitute drill is about 0.007" undersize, so the real question is whether or not your tube will fit in an 0.007" undersize hole. If the tube fits, then the 7 thou tolerance is OK. If the tube doesn't fit, then the tolerance is not OK.
Where the question of tolerance gets more critical is when you are proposing an oversized bore. Clearly a tube will fit in a 0.007" oversized bore without any problem, whatsoever. The question there is whether that oversize gap is too big to give you a good strong glue joint. I don't know what the specs are for the width of the glue line for the three commonly used pen adhesives; but would be willing to bet there is a member here who does. What I do know is that when different kit manufacturers specify different drill sizes
(like a letter drill vs. a metric one) the difference is usually
2-3 thousandths Whether that gives us a clue about the needed thickness of the glue line, I just don't know.
For those who are interested in having an easy reference to see how the various drill systems compare, order a catalog from www.hutproducts.com . There is a very useful chart in the middle of the catalog that shows the comparison of fractional, letter, wire gageand millimeter sized drills. Maybe someone with a scanner could get permission from HUT to post the chart here??? And if you are more atuned to electronic references, check out this site http://www.carbidedepot.com/formulas-drillsize.htm .