Well, beyond that...wouldn't be wiser to ask her to help pick the ring design with you? That creates a bit of a buy-in before you plunk down the money?
Also, promise rings...desperation beyond belief! It is the down payment on the down payment for her heart. Plus, it is driven by some kind of insane belief that guys like me are deterred from hitting on a woman with a promise ring.
Hell, engagement rings often mean nothing. I dated three women who were engaged and then broke their engagements to be with me. It means NOTHING, dude!!! LOL Like I am going to really like a woman and then think twice because she's wearing a ****ing "promise ring" ? Give me a break, man.