In the vanilla game, render scaling is set at:
Low = 70%
Med = 80%
High = 90%
Ultra = 99%

Increments of 25% are more aggressive and also more accurate for rendering on specific monitor sizes. For instance, on a 1440p monitor, a render scale of 75% will keep your UI looking crisp, but the 3D visuals render at effectively 1080p. This makes sense for a lot of low/medium end graphics card. Same for 4k. A render scale of 50% results in a 1080p 3D render resolution.

This mod applies the follow settings, and says the render scale percentage in the UI button.
Low = 50%
Med = 75%
High = 100%
Ultra = 125% (just in case you have extra horsepower and want to effectively overrender the game... I don't know)

This is going to be a part of a set of mods I will be creating to potato-ify the game

To install:

1. Install the Community Mod Loader
2. Download the VMZ file and add to the mods/ folder

Note: the setting will NOT appear when the game is first loaded. When you start a run, the setting will appear