24F08KL302 LPRC running at MUCH lower freq than nominal 31kHz

BlindedByDaLight
Is my math wrong?

Nothing wrong with your math.  I dug up a PIC24F16KL402 (that I had evaluated some time ago but never used in a project) and plugged it into my breadboard.  With your WDT settings I got a timeout of 1.92 seconds, which is about 91% of the nominal 2.11 seconds predicted by your math and WDT settings.

A couple of comments:

• I have never used a WDT as a "precise" timer.  I have used a WDT to try to recover after some "glitch" (peripheral hardware failure; transient electrical problem; cosmic ray; etc.) Exact timing was not an issue.
• For test purposes I can't think of a single reason to run a 16-bit PIC with a master clock of 31 kHz.  I usually run at full speed (32 MHz for this chip), but for testing and evaluation I sometimes use the fastest speed that I can get from configuration pragmas without having to write to registers. (But see Footnote.)

Anyhow...
Here's my test program. Note that I put what I think is pretty much an absolute minimum amount of code so that the timing measurement wouldn't be confused by user initialization code or other user code.

// PIC24F16LK402 WDT timing test program
// Clock 16 MHz:
//  FRC (with default RCDIV) + PLL

// Connections
//  PIC24F16KL402 28-pin DIP
//       LED2 is RB1 (Pin 5 of the DIP28)

// PIC24F16KL402 Configuration Bit Settings
// FBS
#pragma config BWRP = OFF               // Boot Segment Write Protect (Disabled)
#pragma config BSS = OFF                // Boot segment Protect (No boot flash segment)

// FGS
#pragma config GWRP = OFF               // General Segment Flash Write Protect (General segment may be written)
#pragma config GSS0 = OFF               // General Segment Code Protect (No Protection)

// FOSCSEL
#pragma config FNOSC = FRCPLL           // Oscillator Select (Fast RC Oscillator with Postscaler and PLL (FRCDIV+PLL))
#pragma config SOSCSRC = ANA            // SOSC Source Type (Analog Mode for use with crystal)
#pragma config LPRCSEL = HP             // LPRC Power and Accuracy (High Power/High Accuracy)
#pragma config IESO = ON                // Internal External Switch Over bit (Internal External Switchover mode enabled (Two-speed Start-up enabled))

// FOSC
#pragma config POSCMD = NONE            // Primary Oscillator Mode (Primary oscillator disabled)
#pragma config OSCIOFNC = ON            // CLKO Pin I/O Function (Port I/O enabled (CLKO disabled))
#pragma config POSCFREQ = HS            // Primary Oscillator Frequency Range (Primary Oscillator/External Clock frequency >8MHz)
#pragma config SOSCSEL = SOSCHP         // SOSC Power Selection Configuration bits (Secondary Oscillator configured for high-power operation)
#pragma config FCKSM = CSECME           // Clock Switching and Monitor Selection (Clock Switching and Fail-safe Clock Monitor Enabled)

// FWDT
#pragma config WDTPS = PS512            // Watchdog Timer Postscale Select bits (1:512)
#pragma config FWPSA = PR128            // WDT Prescaler bit (WDT prescaler ratio of 1:128)
#pragma config FWDTEN = ON              // Watchdog Timer Enable bits (WDT enabled in hardware)
#pragma config WINDIS = OFF             // Windowed Watchdog Timer Disable bit (Standard WDT selected (windowed WDT disabled))

// FPOR
#pragma config BOREN = BOR3             // Brown-out Reset Enable bits (Enabled in hardware; SBOREN bit disabled)
#pragma config PWRTEN = ON              // Power-up Timer Enable (PWRT enabled)
#pragma config I2C1SEL = PRI            // Alternate I2C1 Pin Mapping bit (Default SCL1/SDA1 Pins for I2C1)
#pragma config BORV = V18               // Brown-out Reset Voltage bits (Brown-out Reset at 1.8V)
#pragma config MCLRE = ON               // MCLR Pin Enable bit (RA5 input disabled; MCLR enabled)

// FICD
#pragma config ICS = PGx1               // ICD Pin Placement Select (EMUC/EMUD share PGC1/PGD1)


#include <xc.h>

// LEDS: LED1 is RB1 (Pin 5 of the DIP28)

#define LED1_TRIS TRISBbits.TRISB1
#define LED1 LATBbits.LATB1


// FRCPLL with default RCDIV of 1:2 gives system clock = 16 MHz
#define FCY   8000000UL
#include <libpic30.h>

int main()
{
    LED1 = 0;
    LED1_TRIS = 0;
    __delay_ms(100);
    
    ClrWdt();
    LED1 = 1;
    Sleep();
    // XC16 generates return to a break;reset sequence
    // or you could have the following
    //asm(" reset");
    return -1;
} // End of main()

Result: LED periodic output was On for 1.92 seconds and Off for 100 milliseconds.

Regards,

Dave

Footnote:
I changed the system clock to be LPRC as you had it.  Results were about the same: Still got 1.92 second WDT timeout.