Your stock wheels are between 57 and 45mm positive offset. The oem 18's are 57mm, so lets add 12mm (your -12 wheels) to the oem 57mm = 69mm / 25.4mm per inch ~ 2.72". Keep in mind, this is how much the center of your tire tread will move out. You will also need to factor tread width to figure out how far the outer tread edge will move out, this is usually where the interference will be. For example, if your stock tire has 9" of tread width and your new tire has 10" of tread width, the outer edge of the tread will move out and additional 1/2" inch. With the new wheels and tires, the outer edge will move out roughly 3 1/4" ...
From my experience, anything more than about 15mm will rub severely with a 35x12.5 tire and no trimming. The 15mm positive offset will move your tire centerline out by about 1.65"
When you say 11.5 to 12" tire, I think the 285/75r18 is about 35" and 11.22" overall width, this tire will have less tread width than a 35x12.5 (my 35x12.50r18 Toyos had almost 11" of tread width) and therefore better clearance with wheels sticking out further, such as the -12 wheels you mention.
