I always thought the rule-of-thumb was a months money.
My only advice to anyone buying rings, either diamond or gold, is never buy on the high street.
The lack of knowledge, here in the UK, is abysmal. I studied the subject before heading off with me other-half, and read up on the "Four C's" and wanted to discuss the 'spec' of the diamond.
I might as well have been speaking Swahili. In several mainstream, national jewellers, I couldn't get the info I requested and almost got queried as to why I wanted to know. Simply astounding. Its like I wanted something secret.
Later the next year, we went as a couple to buy wedding rings. A similar experience.
So...
We went to the Jewellery Quarter in Birmingham, where there is street after street of actual jewellery manufacturers. I looked at the way they sell the gold rings and they know whats in their kit. Why? Because they're making it.
The worth of a gold ring is all about weight. An alien concept to high street jewellers so it would seem.
A story for you...
Two years ago, my wife lost her wedding ring. Was gutted. We had the receipt and the ring details so we thought it was an easy job to go into town, and trail about for a ring to the same spec.
She had a white gold ring of 4.5g.
Not one jeweller of ten understood the concept of worth by weight. Staggering. Several didn;t even have jewellery scales, or understood the significance of wanting to know the weight, The best quote I could get was £800.
We went back to the original dealer and had them remake the same ring....£225.
Shocking markup.
Never, ever buy a ring from the types of main street, city centre chainstore jewellers. Seek out an actual jeweller. Know your subject.