timing

Ok, i thought i understood timing but now i dont really get this part.It is a msd system.
At sea level, my cam specs are for i belive 38 degrees timing. I moved up to 4000ft and i have it at like 31 or 32 right now at initial timing.
Now my question is, when i hit about 2500 rpms, my timing is at like 42. What exactly does the 42 mean? I bumped the timing up a few degrees yesterday and ran really good but it scared me cuz when i revved it up it was saying like 45 degrees. Am i doing something wrong or what does this mean? Its also vacuum adavance.

thanx
david

Sometimes the folks at AF don't like double posting the same question in 2 different sub forums.
However, check out my 500 word reply in the Engineering/Technical forum. :)
http://www.automotiveforums.com/vbulletin/showthread.php?p=3347832&goto=3347832

sorry, since its a hp engine i thought id post here too. didnt know if one would be a better spot than the other.