Hey,
There is no "right" amount to spend on an engagement ring. Everyone has a different budget, so everything you have heard (25% of annual income or three months of annual salary) are just suggestions or traditions.
When buying an engagement ring, it is important that you stay within your budget. I would suggest checking out online engagement ring sites like James Allen. James Allen has an impressive selection of quality engagement rings at all prices. They also have a wonderful customer service team to answer all your questions.
Definitely check out James Allen,
http://www.jamesallen.com/.
Hope this helps,
Judy