What is the general rule of thumb regarding UL's creepage clearance regulation?
The important thing to remember about this sort of things is that these are ratings. Ratings are about safety in the edge cases of a big mains spike and heavy board contamination and high humidity and ... you get the idea.

Ratings make all sorts of assumptions about the expected safety level required, the likely board contamination, the likely spikes expected on mains supplies (1500v is not uncommon!) etc. This is why you should not expect exact agreement between UL figures, and a website doing calculations to IEC60950. Slightly different standards, slightly different assumptions. 

You may think you can scale between 300v and 220v? For Ohm's law, yes. For ratings, not really. Ratings tend to specify bands, though they'll sometimes have formulae.

Obviously the safest way is to calculate using both UL and IEC60950, and choose the largest. You say you haven't the space for that. It's quite common to see a routed slot in a PCB underneath things like optocouplers and transformers used for isolation. This increases the creepage distance to the perimeter of the slot rather than the diameter. The clearances for air and creepage surface will usually be different. Read the standard carefully.

So the next question is, is this a one-off for personal use, or is this a commercial product, that you hope to sell?

If you are going to sell this thing, then you need to find the space for the rated clearance. Your product won't get approval, and you'll find it difficult to sell without. If a customer's factory burns down, you may get sued by them, as their insurance would be void for using non-rated equipment, even if it wasn't yours that started the fire. 

If this is a one-off for personal use, then the previous paragraph doesn't apply. Obviously, larger is better. It's quite possible to make something that works, but would not get approval under UL or IEC60950. The risk is yours.