Print with round towards zero instead of round to nearest
A cheap trick to round a number to one decimal place (in "chop-off mode") is to multiply it by 10, convert it to an integer, and divide by 10.0f again:



float myVal = 123.89f;
myVal = (long)(myVal * 10) / 10.0f;
// = ((long)1238.9) / 10.0f
// = 1238 / 10.0f
// = 123.8


You can then print this modified number to the serial monitor.

This method can also be generalized to other precisions. For 2 decimal places, we just have to multiply by 10^2 (100) and divide by 10^2. A macro would be:

#define TRUNCATE(val, numDecimalPlaces) \
    (float)(((long)((double)(val) * pow(10, (numDecimalPlaces) * 1.0f))) / (pow(10, (numDecimalPlaces)* 1.0f)))


Note: You don't have to actually dynamically compute pow(10, numDecimalPlaces), you can also create macros for each decimal precision from 1 to x and simply precompute that value (100, 1000, 10000, ..).

The same can also be achieved by using the standard library function trunc from math.h:

#define TRUNCATE_TO_ONE(val) \
    (trunc((val) * 10.0f) / 10.0f)


See http://www.cplusplus.com/reference/cmath/trunc/.

Here's a small sketch in which it is demonstrated:

#include &lt;Arduino.h&gt;

#define TRUNCATE(val, numDecimalPlaces) \
    (float)(((long)((double)(val) * pow(10, (numDecimalPlaces) * 1.0f))) / (pow(10, (numDecimalPlaces)* 1.0f)))

#define TRUNCATE_TO_ONE(val) \
    (trunc((val) * 10.0f) / 10.0f)


void printValueFormatted(float value, int numDecimalPlaces) {
    //format correctly
    char buf[20];
    char* res = dtostrf(value, sizeof(buf)-1, numDecimalPlaces, buf);

    //skip over empty chars
    while(*res == ' ')
        res++;

    //print buffer
    Serial.println(res);
}


void setup(void)
{
    Serial.begin(115200);
}

void loop(void)
{
    float myVal = 123.8978f;
    printValueFormatted(myVal, 4);
    printValueFormatted(TRUNCATE(myVal, 1), 1);
    printValueFormatted(TRUNCATE(myVal, 2), 2);
    printValueFormatted(TRUNCATE(myVal, 3), 3);
    printValueFormatted(TRUNCATE(myVal, 1), 4);
    printValueFormatted(TRUNCATE(myVal, 2), 4);
    printValueFormatted(TRUNCATE(myVal, 3), 4);
    delay(1000);
}


Prints 

123.8978
123.8
123.89
123.897
123.8000
123.8900
123.8970