When you locate the wet darkroom, be sure to consider what useful alternative use it might serve in the near future. Sorry, I'm a smart a**.
As long as you're cutting mattes etc, why not also make frames? Which means a miter saw. Which should always be located so that whatever airflow you have in the structure exits through that area carrying fine sawdust with it to the outside.
30 x 50 is big enough. The spanning structures that support the roof should be off a type that is friendly to clamping on lights, parts of sets, large hanging reflectors etc. Most industrial spaces already have them. 13 feet is high enough that the roof can be white. Lower roofs should arguably be black so as not to intrude on your lighting designs.
Your Photoshop area should have very easy access to the stage area. The door should be wide enough so that you can roll the entire Photoshop desk out onto the stage for tethered shooting.
If possible locate the stage part of the building so that it is on the north or east side of the building. That will slightly mitigate ugly solar temperature build up when you are working with hot lights. Also locate the stage garage door in a location that does not receive direct sunlight. If you install turbine air exhausts on the roof, you need to hang 4 x 4 foot black light baffles underneath them. If you install air conditioning don't put in on the roof, because it will make those place bounce up and down and sound like factory.
Also be sure to locate the coffee and client diversion area with easy access to the stage, but decidedly not ON the stage. When clients get bored enough they'll go there and you'll be able to get some work done. A TV or internet station can be helpful in that regard.
Don't forget dressing rooms with big mirrors and counters just underneath. Potty, wash basin, and shower included.
Install linoleum tile over concrete floors, or a least paint them really good.
Be sure there a couple foam pads in the cupboards for late night crashes. Also a frig full of brews and Diet Coke, and at least a microwave. Cuisine grade gas stove, hot tub, etc are optional but recommended.
You also need to buy several mural sized pieces from me to hang on your walls.
It is easy to predict exactly how much space you need. It will always be a little more than you have. Clutter always expands to exactly overflow the space available.