I added lines so you can tell where the top and bottom of the soil are. Blue is the top, showing how it is sloped from back to front. Red is the bottom of the soil with the lava rock below.
What kind of plant are you going to add to the terrarium? Moss and epiphytic plants don't need much soil, while "normal"(terrestrial) plants need different soil thickness depending on how deep the roots grow.
In addition, with a terrarium of such size, I would recommend having a thicker drainage layer. Larger terrariums need more water to create the high humidity; a thick drainage layer could store more water without soaking the soil.
I bought a random mix of terrarium plants online. Plus I need to grab some moss before it gets too cold out. I'll post pics of the combleted terrarium soon.
