what lasts longer a tyre or a chain? For myself i find the winner by many miles is the front tyre. the rear tyre and chain are usually close to each other.
Broadly my experience too, when I fail to change a chain in time and run it on till the cassette needs replacing at the same time. Mostly, though, I change my tyres when they still have many miles in them but I have a very long audax to complete (and failure because of a failed tyre is a risk not worth taking). Last chain: 5269km and this one 5245km (and not skipping yet). Suggest your 'rule of thumb' depends on running decent tyres (eg GP4S or Pro4s). Rough ratio of front to rear tyre longevity = 8/5 ime.
Currently running odd wheels and odd tyres (terminal damage to front rim and tyre in major off (September)). Have replaced and relaced new rim so will get back to matching, as and when.
I have previously swapped a used front to the rear but would never put a used tyre on the front - well discussed on a recent thread. A failed rear tyre is a pain. An abruptly failing front tyre can be the cause of an accident.