mash temperature and time effects on final gravity.

I am getting pretty good with a basic understanding of brewing and I am really trying to understand a few things that I can't wrap my head around. My understanding is higher temperature + shorter time = higher final gravity and lower temperature + longer time = lower final gravity. Is this correct? If so is there some way to know the effect? For example what would be the effect on a 1.040 OG brewed at 155° for 60 min as opposed to 150° for 90 min?

In my experience, its not so much a matter of time as in temperature. Mashing at 155 will definitely leave you with a higher FG then mashing at 150. I recently did a IPA that the first time I mashed at 154. It went from 1.068 to 1.012. The same recipe a couple of months later mashed at 149 came in at OG 1.069 to 1.008. Beersmith can give you a guesstimate, but for me it comes from knowing the yeast strain and knowing your system. Repetition is the name of the game for me. At least that is how I'm still learning. You want mouth feel, go for a higher mash temp (stouts and porters and such). You want dry and higher ABV?....lower mash temp.

Somewhere on HBT, I once saw a link to this, it explains it in pretty good detail.

Longer time in the mash (to a point) is a compensation for the type of crush necessary in a conventional mash tun as the grain particle sizes may be a bit large and the water needs time to reach the center of the particles and draw out the sugars produced by mashing. The actual conversion time is fairly short. Mash temperature is what decides how fermentable the wort will be.