maximum distance between IC power pins and decoupling capacitor
Nobody can specify a maximum distance!

Even if in the datasheet 2mm is mentioned, doesn't that mean, that the chip won't work with 3mm. You won't even recognize a performance degradation or something like that in most cases. 
The longer the trace, the more will your supply drop. Currents are often not high, so thicker traces doesn't solve your problem sometimes. A bad design is often not that good in EMI measurements, if you have the possibility for a test.

Actually you can guess a little bit...
Maybe you have output rise and fall times for your IC, you could calculate the resulting frequency and calculate the impedance of the trace. But again, nobody will tell you a maximum impedance, so do best effort.

However, you wrote you are designing a single layer board. Most of this boards i saw had the same mistakes: Everyone places a capacitor directly on a VCC pin, but the current comes back through the GND pin of the IC. So don't look for the nearest space for 100nF, look for the smallest current loop through the VCC AND GND pins. Standard logic devices don't have a very good pinout in my cases, because the supply pins are far from eachother.

If you aren't making your PCBs at home, consider to make more layers. More layers aren't soooo expensive this time and you get an much better design.