How much should you spend on an engagement ring?
There is no rule. "Two months' salary" was written by an advertising agency for De Beers in the 1930s, and "three months" was the 1980s update. What follows is arithmetic instead: what your money can carry without debt, and what that buys.
Your numbers
Nothing is stored or sent anywhere. The arithmetic runs on your device.
What the sensible figure buys
Priced with our model at today's rates. Natural stones resell for 25–40% of retail; lab-grown for 5–12%.
The honest version
A ring is a purchase, not an investment: whatever you pay, a buyer will offer you a third of it back at best. That is the strongest argument for two things — never financing it at interest, and spending on what is visible (carat and cut) rather than on grades that need a loupe. It is also the argument for lab-grown if the stone is meant to be worn rather than kept: the same money buys a stone two to three times the weight.
The figures above are general arithmetic, not advice about your finances. If the sensible band looks impossibly low, the answer is usually a smaller stone in a beautiful setting, not a loan.