What percentage of screen power usage is healthy? Using GSam, the screen uses about 50% of power compared to apps. Is this normal/healthy?
Yes, this is completely normal especially on high-resolution devices.  It takes a lot of juice to produce all that light, switch colors, etc.  Activities like watching videos will exacerbate this.

The only times you will usually see the screen not use the most battery is if you have the screen off for a long time, or if you play a really intensive game.