When Wideband is Heating up, how does MS calculate AFR?
 

I have MTX-L and using it as only O2 input to MS. (Installing tomorrow)

It takes about 15s to warm up, what does the MS do in that time frame?

Currently, the stock ecu when car is started, will be around 13 and inch up to 15ish over the first minute of idling.

whatever you tell it to do

there is cranking, after start and warm up enrichments that you can control. you would also have any closed loop corrections set to off until the vehicle comes up to temp. if everything is set correctly, it will ignore the 02 until long after it has warmed up.

In case you were also interested in knowing what the gauge is reporting. You can also program the MTX-L what to report during warmup and error conditions.

As per the manual for Innovate Motorsports Logworks:
The ‘Advanced’ button allows the user to set the analog out update speed and the voltage output during sensor Warm-up and Error Condition. The factory defaults of the analog outputs update 1/12 of a second. The default voltage output is set at 0 volts for both the Warm-up and Error Condition.