MSP430p2553 and Timers

Hi,

I am preparing for an exam and am having a hard time wrapping my head around timers. I have (simple) working code that alternatively blinks the red and green LED approx. every second using a timer. I am using TASSEL_1 which is the ACLK clock. This clock runs at 32 kHz. I set the timer to be an UP timer and am counting to 200 (well, the interrupt is occurring every 201 counts :o) and this is what confuses me -> the interrupt is happening about every second but the way I got there was with playing with the TACCR0 value until it seemed ok to me. So how do I calculate the correct value for TACCR0 and not just guess it? 

I do realize that this has been explained about 1000 times online but I just cannot seem to understand any of the examples (http://www.msp430launchpad.com/2010/07/timers-and-clocks-and-pwm-oh-my.html, for example, explains it quite well but not for the ACLK). Any help would be greatly appreciated.

Thanking you in advance

Ursus

------------------------------ here is the code

#include <msp430G2553.h>

// just some defines
#define BOOLEAN int
#define FALSE 0
#define TRUE !FALSE

// my two LEDs are on BIT0 and BIT6
#define RED_LED BIT0
#define GREEN_LED BIT6

// set when the RED LED is on
BOOLEAN led_red = FALSE;

/*
 * main.c
 */
int main (void) {

    WDTCTL = WDTPW | WDTHOLD;	// Stop watchdog timer

    // setup the timer
    TACCTL0 =						// Capture/Compare Control Register
    		CCIE;					// interrupt enable
    TACTL = 						// Timer_A Control Register
    		TASSEL_1 + 				// = ACLK is a 32kHz crystal clock
			MC_1;					// Use the UP mode timer (set up count in TACCRO
	TACCR0 = 200;					// Timer_A will count UP to here

	// setup LEDs
	P1DIR |= RED_LED + GREEN_LED;	// set P1DIR out for RED and GREEN
	P1OUT &= ~RED_LED;				// turn off red
	P1OUT &= ~GREEN_LED;			// turn off green

	// start interrupts
	_enable_interrupts ();

	while (1==1) {
		// do nothing
	}
}

#pragma vector=TIMER0_A0_VECTOR
__interrupt void timer (void) {

	if (led_red == TRUE) {
		led_red = FALSE;
		P1OUT &= ~RED_LED;			// RED OFF
		P1OUT |= GREEN_LED;			// GREEN ON
	} else {
		led_red = TRUE;
		P1OUT |= RED_LED;			// RED ON
		P1OUT &= ~GREEN_LED;		// GREEN OFF
	}
}