That's a little powerful to run in a stock G18C - comparible to Nuprol 3.0/4.0 and Guarder Powerful. Maybe switch to a less compressed gas after you're done with that can. It doesn't take much to make that plastic slide really snappy
Don't run .12g as they have no range because they shed their energy early during flight and don't hold the inertia of the hop as long. In fact, you should invest in some quality .25g, not .20g - part of the great thing about Maruis is they shoot slower due to Japanese laws but go super far because they have a decent hop out of the box. Dialling in some .25s is a good idea as you'll get 35+ metres out of it that way.
Remember, remove the slide and use the hop wheel. Turn it to a very low setting, shoot it, then if the BBs dip too much (which they should on the first try) take the slide off and turn the wheel a little before shooting again. Rinse and repeat until you get a flat trajectory. If your BBs curve into the sky then you've put the hop on too much.