Hello there,
A good tip to get is to get under the hood and understand what those presets are doing. When changing a preset to low, high, medium, you’re actually loading and unloading a collection of console variables. Console Variables in C++ | Unreal Engine Documentation
You can go in Window > Consoles Variables
Once the window pop up, you can see what values are beeing edited by each preset. Tweak what you need, then save it as a preset, you’ll be able to reuse that preset in editor, rendering or in game.
Hope that helps. 
T
