Do you compensate for slides-outs when leveling first?

Our 5er sags when the slide goes out. I can never figure out how much bubble to allow for ahead of time (there is a trick people use for that, but I find that too difficult to learn) and how many planks under the tires that would be equivalent to.

So the simple thing to do is run out the slide first, play with the planks under the tires back and forth until the bubble is in the middle, then lower the front jacks and unhitch the truck, then level fore and aft using the other bubble.