Change offcanvas trigger point

I'm using SP Pagebuilder with Helix framework - all latest versions. I need to change the trigger point for the offcanvas menu so that it changes to offcanvas sooner. Because my menu is quite wide it is OK on laptop, but on tablet landscape the menu gets shifted down into the main body. At present it appears that the trigger point is set at a width of approx 992 pixels. However I need the offcanvas to appear at something like 1200 pixels. What do I need to change via custom.css in order to make this happen? Can't find anything about this in documentation.

https://ibb.co/VWRTz0bN