After reconsidering this problem, I found that there are limits to how fine the LCD Setup Menu adjustments can be easily made. This is because the settings are saved to limited EEPROM space and the fact that the associated variables 'minval', 'maxval', and 'offset' are saved in EEPROM space as 8-bit integers.

See in header file setup.h:

typedef struct {
	const char* formatstr;
	const NVItem_t nvval;
	const uint8_t minval;
	const uint8_t maxval;
	const int8_t offset;
	const float multiplier;
} setupMenuStruct;

As an example, uint8_t is 1-byte unsigned with range of only 0 to 255, while int8_t is 1-byte signed with range of only -128 to 127.

Here are some acceptable changes that increase the resolution of LEFT and RIGHT TC's, for both GAIN and OFFSET values, configured in the LCD Setup Menu, with the following changes made to file setup.c.

static setupMenuStruct setupmenu[] = {
	{"Min fan speed    %4.0f", REFLOW_MIN_FAN_SPEED, 0, 254, 0, 1.0f},
	{"Cycle done beep %4.1fs", REFLOW_BEEP_DONE_LEN, 0, 254, 0, 0.1f},
	{"Left TC gain    %1.3f", TC_LEFT_GAIN , 150, 250, 50, 0.004f},    //  <-------
	{"Left TC offset  %+1.1f", TC_LEFT_OFFSET, 0, 250, -125, 0.20f},   //  <-------
	{"Right TC gain   %1.3f", TC_RIGHT_GAIN, 150, 250, 50, 0.004f},    //  <-------
	{"Right TC offset %+1.1f", TC_RIGHT_OFFSET, 0, 250, -125, 0.20f},  //  <-------
};

Note the blank spacing changes in the text labels, before the "%", as this changes the position of the displayed text on the LCD Setup menu. Changes also corrects for the current inverse texting artifact at the far right of the display, when LEFT or RIGHT TC OFFSET values are too negative (compare the two images shown below).

The above changes will allow setting:
Left and Right TC Gains in range of values between 0.800 and 1.200 in increments of 0.004, and
Left and Right TC Offsets in range of values between -25.0 and +25.0 in increments of 0.2 degrees C.

Setup Menu with above changes..

For comparison, Setup Menu with stock firmware V.0.5.2..

To get the above changes to work with the oven TC sensors, some additional changes are needed in files setup.c, setup.h, and sensor.c. Similar to what's done in the setup code file, the sensor code file also converts RAW data values read from EEPROM memory, to the real data values ( i.e., the Left and Right TC Gain and TC Offset ) so that they are consistant with what's shown on the LCD display Setup/Calibration Menu, using the data conversion parameters (i.e., offset and multiplier). The conversion parameters were hard coded in the sensor code file too, so rather than editing the hard coding in two separate files, the following changes eliminate the need for that.

Specifically, in file setup.c,

change line 26, as follows..

    const setupMenuStruct setupmenu[] = {

In file setup.h,

add a new line after line 12, as follows..

    const setupMenuStruct setupmenu[6];

In file sensor.c,

add a new line after line 11, as follows..

   #include "setup.h"

Also in file sensor.c,

change the following lines,

line 44, as follows..

   adcgainadj[0] = ((float)temp + setupmenu[TC_LEFT_GAIN].offset) *
						setupmenu[TC_LEFT_GAIN].multiplier;

line 51, as follows..

   adcgainadj[1] = ((float)temp + setupmenu[TC_RIGHT_GAIN].offset) *
						setupmenu[TC_RIGHT_GAIN].multiplier;

line 58, as follows..

   adcoffsetadj[0] = ((float)temp + setupmenu[TC_LEFT_OFFSET].offset ) *
						setupmenu[TC_LEFT_OFFSET].multiplier;

line 65, as follows..

   adcoffsetadj[1] = ((float)temp + setupmenu[TC_RIGHT_OFFSET].offset ) *
						setupmenu[TC_RIGHT_OFFSET].multiplier;

That is all.