Is there a standard target level for spoken audio in a tutorial video?
There's no single answer - it's a series of trade-offs:

When you record your voice, it should be as loud as possible, but making sure never to go over the maximum level that can be recorded.  This is known as clipping, and sounds distorted.  Too quiet, and your audio will be hissy relative to the noise floor of your setup - too loud and it will clip.

In order to smooth out the overall levels, you can dynamically compress your audio (this is not the same as data compression), either before it is recorded (between the mic and recorder) or after it has been recorded.  This is effectively like having an automatic volume control, where levels over a certain threshold are turned down, so that the overall effect is smoother.  Too much compression can sound pumping and aggressive - like an FM talk-radio host.