I've had this question asked a lot, what makes a good light meter? Well first off, toss out any idea of using a light meter from your phone. I've never found them to work very well. Any stand alone version seems to work just fine, it's how you use them. A light meter that has a white globe to difuse direct lighting is why the stand alone units work the best. If the standalone unit is just a lens dont buy it. You can find these meters at your local electrical shop for around $100.
Ensure you're standing as far back as you can as you'll impact the light levels if you're in the way of the view. Most units have a nice cord length so you can step back. Don't buy one without this feature!
Take various samples and jot them down on your drawing in the location taken.
I hope this helps. leave a comment if you have any questions.