Comment by zamadatix

If you mean https://i.imgur.com/0LtYuDZ.jpeg that is probably the slider GP wants but it's not about matching HDR white to SDR white, it's just about clamping the peak HDR brightness in its own consideration. The white on the left is the HDR brightness according to a certain value in nits set via the bottom slider. The white on the right is the maximally bright HDR signal. The goal of adjusting the slider is to find how bright of an HDR white your display can actually produce, which is the lowest slider value at which the two whites appear identical to a viewer.

Some games also have a separate slider https://i.imgur.com/wenBfZY.png for adjusting "paper white", which is the HDR white one might normally associate with matching to SDR reference white (100 nits when in a dark room according to the SDR TV color standards, higher in other situations or standards). Extra note: the peak brightness slider in this game (Red Dead Redemption 2) is the same knob as the brightness slider in the above Battlefield V screenshot)