Bandwidth Settings for Inconsistent Cellular

I am using a Peplink Balance 20X to bond two cellular connections using external modem/router devices. WAN will be connected to a MicroHard BulletCAT11, with a SIM from a local MVNO that provides access to AT&T’s LTE network. USB is connected to an Ethernet Dongle, which connects to a T-Mobile 5G Home Internet device. Both have provided over 100 Mbps down (very rarely), but I can normally get about 40 down from AT&T and 25-30 down from T-Mobile. Dependable upload is closer to 20 & 10-15.

I want to set the Upload and Download Bandwidth in a way that gives SpeedFusion the greatest chances of success. The manual just says to ask my ISP for these settings, but both of my Cellular ISPs only give me “what they can at the time”. It seems counterproductive to specify the maximum speed of the Gigabit Ethernet interface, or even the maximum theoretical speed of the devices.

I guess my real question is: When a given link is known to provide wildly varying speeds (such as under 10 to over 100 Mbps, with “normal” somewhere around 20) how do I decide what to set this value to?