Add a cup or so, stir well, take OG. If it is where you want it, good, if not add more repeat.. Sorry just did a search on a calculator but couldn't come up with anything useful.
I want to say that for a 5 gallon batch you would need about 6.5lbs of LME to get close to your 1.045. Or at least that is what has come with kits I have bought with similar OG.
Not sure if this helps, but it will get you through in a pinch.