How can I keep the resolution slider within a useful range?

Open Settings > Displays > [display] > Additional Settings… and enable Limit resolution slider and keyboard shortcut range. Set the minimum and maximum desktop dimensions you want the slider and resolution-adjustment shortcuts to use.

Width and height have separate limits. Leave a field blank when you do not need that bound. For example, setting Min Width to 1512 and Min Height to 982 excludes smaller logical desktop sizes from those adjustments. Keep each minimum below its maximum.

These limits filter existing choices. They do not create or remove macOS display modes. The current mode remains in the slider even if it falls outside your bounds, so choose an in-range mode if needed.

Customize Flexible Scaling Resolution Range has a different purpose: it sets the range of resolutions BetterDisplay generates. Additional variants and system-provided modes can still exist outside that range. Use the slider-limit setting above to narrow everyday adjustments. See Additional Settings for the range fields and warnings.

For the underlying rendering method, see what flexible scaling and custom resolutions change.