If cooking multiple dishes in the oven, how much cooking time should I add to compensate for heat loss every time I open the door to add a new dish?

Thanks, everyone. I kept reading that you lose SO much heat every time you open the oven door that I was getting worried. I think my best strategy will be to preheat the oven well in advance so those walls get hot, like you do when baking bread a high temperature.

I agree with Smaug and Nancy. I don't think you can time everything to come out on point at exactly the right time either mathematically or percentage-wise. The oven walls are so hot during a stretch of cooking that heat retention is better than cookbooks tell you. Also, washing, slicing, dicing and prepping the next dish that's destined for the oven always takes longer than you think so although a general time frame is helpful, don't fret if you can't strictly adhere to it.

I very frequently have to cook multiple dishes in the course of a single day as we very frequently host guests. Cook with your senses and be aware of the "good cooking smells" (as I used to call them when my kids were younger) emanating from your oven. Sometimes there's a fragrance that tells you a dish is done and it's time to open the oven door even as you're immersed in preparing the next item, so be aware and cook with your senses.
I would add, prep, cook or assemble what you can in advance (washing, peeling, dicing, or even cooking and puréeing vegetables and storing them in zipper bags, the night before the actual cooking, measuring out flour and dry ingredients or cubing butter for baked goods and storing them in zipper bags also helps. You can even grease and flour pans and line them with parchment paper the day before baking, covering them with a clean towel, a piece of plastic wrap or a same-size dinner plate). Some things like cranberry sauce actually benefit from cooking a day or two in advance so the flavors can mellow. Other things will not be the worse for making them ahead of time even though you may want everything "perfect." Even if it's just scrubbing the skins of winter squash the night before, every bit of advance prep helps!

The heat loss is really not very significant if you don't leave it open for long periods- the air itself holds very little heat compared to the oven walls, racks, and whatever you're cooking, so if all you're losing is hot air it shouldn't make a big difference.