Also have a list of things you'd be willing to accept instead of more money (if they really can't do it) i.e. paid parking, more time off, work from home days
Depending on where you work and how pay scales are determined they may be able to give you additional perks instead.
Also, keep in mind that you have already proven yourself able to do the job. Sometimes they use "need to prove you can handle the additional responsibity/type of job" as a reason not to increase your pay when you start.
in your case, you have spent 2 years proving you can do this,
You may have more power than you think. They definitely do not want to train someone to take over when you are already trained (otherwise they would be looking for that PhD). You could always be looking elsewhere, now that you have this experience under your belt.
I think you have more leverage than you think, because you already know how to do the job. A proven employee is worth so much more than an unproven candidate. You may also have more work experience than a PhD. In my field, PhDs are paid more, but a new PhD is paid about the same as a person with an MS + 4 years experience. So don't value yourself less than a PhD, because you have proven you can do the work.
Ok, was just curious if it was public knowledge or something that you could refer to.
Yes, rule I've always been told is to always ask!
My FI was in a similar situation, his boss left and he was doing his boss' job for about 4 months before he was offered the promotion. He was offered a salary which was in line for what others make in his field, but he did ask for more on top of that. Unfortunately, they said no to him, but his manager was really supportive of him asking and agreed that you should always ask.