It's not based on OG, it's based on style and personal preferences. When I'm designing a recipe, I look at the style guide. The BJCP style guide is loaded in BS. Just click on "Style" in the "Profile" tab and it will bring up a description of all of the styles.
They are listed alphabetically. The first one is American Amber Ale. When you read down the through the description of the style, it will give you an idea of the body of the beer for that particular style. For an American Amber Ale, it is described as "Medium to Medium-Full Body". You can either use "Single Infusion, Medium Body, and Batch Sparge". You can also use different sparging systems to fit your equipment and techniques, while still using the single infusion, medium body profiles.
That will get you the strike water temperatures and mash temperatures for medium body. If you wanted to go with a medium-full body, because you might want more body to the beer, then you could use either the "single infusion, medium body" or the "single infusion, full body", and then you'd have to go into the "mash" tab within your recipe. To get it from either medium body or full body to a medium-full body, you can alter your mash in that "mash" tab. To alter it in the "mash" tab, just double click on the mash profile description under the word "Description". This will open up a little window where you can precisely dial in the temperatures, water to grist ration, etc.
So let's say that you used, "Single Infusion, Medium Body, Batch Sparge" on the design page of your recipe for your "mash profile". You then open the "Mash" tab. Then double click on the mash profile under the word "Description". You see a target mash in temperature of 152F that your mash profile is trying to hit. This would give you a medium bodied beer. If you had used "Single Infusion, Full Body, Batch Sparge" instead, it would have given you a target mash temperature of 156F, instead of the 152F.
152F would be a meduim bodied beer. 156F would be a full bodied beer. Both would convert the starches to approximately the same amount of sugars. The difference would be that the fuller bodied beer would have more sugars that weren't fermentable by the yeast, thus a little more sweetness, fuller taste, and less alcohol. You'd have about the same starting gravity with both, but your finishing gravity would be lower with the medium bodied beer, meaning that the yeast were able to convert more of the sugars of the medium bodied mash profile into alchohol, thus making a drier, medium bodied beer with less residual sweetness and higher ABV.
So, the question now is, how do you get a "medium-full" bodied beer, when beersmith only gives you a choice of medium or full. It's actually quite easy. Let's say you've opened up the description window in the Mash profile tab of your recipe. Just change your target mash temperature to 153F, 154F or 155F. To fit the style for an American Amber Ale, you'd choose 153F, 153F or 153F. It will automatically change your strike water temperature to compensate for the different desired "mash temp". Anything from 153Fand 155F would be in the medium-full range, although 155F is pretty much a full bodied beer.
You can also do it the opposite way, by using the full bodied desciption and then changing your temperature down to fit what you want to have your beer turn out like.
Basically though, light body mash profiles start you at 148F, medium body profiles start you at 152F and full bodied start you at 156F.
I have a "Template Recipe" set up as my base recipe that all of my recipes are created from. I have it defaulted to medium body and then just adjust up or down from there. If you move your target mash temp all the way down 4F from medium body, it will give you the exact same mash profile that using a light bodied profile would have given you.
I hope that this helped, even though it was long-winded.