Hydro Max brake master cylinder

[bedbug123]

The master cylinder is high up on the fire wall and very hard to see levels. Instructions on master cylinder say fill to RING, but what ring. I have to use a flashlight and mirror see down in side, but it is not reliable to the actual level. What are some "work" arounds? Master cylinder has two caps, does that mean there are two sections? Does filling the front also fill rear?
How do I make sure it is at proper level?
Seems like a measuring stick with a mark on it would work best;

Hurricane 32' V10 GVWR 21,000

[16ACE27]

Same way I check battery water levels - with a mirror and flashlight.