The "3 to 1" rule evolved because it was simple "pilot math" and produced reasonably close results as long as appropriate corrections were applied for winds. Adding another digit is fine if you can do the math and if it produces a more accurate result for your aircraft. In the example above, a still air descent of 30,000 total feet would take about 90 miles vs the FMC calculation of 87 miles. That's pretty close all things considered and the math is easy (keep in mind that you could be busy with other tasks so easy is good). The important point for any of these rules of thumb is to constantly update it during the decent to see how it is working out, so in reality you are doing this calculation multiple times as you adjust the descent along the way.
Now, while I admire your attempt at precision, I would simply point out that the penalty for getting down early is burning a little extra fuel while the penalty for missing a crossing altitude could be an ATC violation. Unless you are really short on fuel, I would suggest that erring to the conservative side might be more in your interests.