ATmega328 fuse settings for 2.4576MHz crystal & optiboot

Hi, I need to use Arduino at lower frequency. I need to set fuses for 2.4576MHz crystal with brown-out disabled. I don't want to brick a ATmega328, so I need help.

Arduino Uno board with ATmega328 @ 16MHz with bootloader fuses: LOW=0xFF, HIGH=DE, EXT=0x05, LOCK=0xFF
http://eleccelerator.com/fusecalc/fusecalc.php?chip=atmega328p&LOW=FF&HIGH=DE&EXTENDED=05&LOCKBIT=FF

ATmega328 factory settings (1MHz internal): LOW=0x62, HIGH=0xD9, EXT=0xFF, LOCK=0xFF
http://eleccelerator.com/fusecalc/fusecalc.php?chip=atmega328p&LOW=62&HIGH=D9&EXTENDED=FF&LOCKBIT=FF

  1. Without bootloader

ATmega328 2,4MHz crystal, brown-out disabled, without bootloader LOW=0xFB, HIGH=D9, EXT=0xFF, LOCK=0xFF
http://eleccelerator.com/fusecalc/fusecalc.php?chip=atmega328p&LOW=FB&HIGH=D9&EXTENDED=FF&LOCKBIT=FF

And writing fuses and firmware (USBtinyISP):

avrdude -c usbtiny p atmega328p -U lfuse:w:0xfb:m -U hfuse:w:0xd9:m -U efuse:w:0xff:m 
avrdude -c usbtiny p atmega328p -U flash:w:firmware.hex
 # or
avrdude -c usbtiny p atmega328p -U lfuse:w:0xfb:m -U hfuse:w:0xd9:m -U efuse:w:0xff:m  -U flash:w:firmware.hex
  1. With Arduino Optiboot bootloader:

ATmega328 2,4MHz crystal, brown-out disabled, arduino optiboot bootloader LOW=0x62, HIGH=DE, EXT=0xFF, LOCK=0xFF
http://eleccelerator.com/fusecalc/fusecalc.php?chip=atmega328p&LOW=FB&HIGH=DE&EXTENDED=FF&LOCKBIT=FF

Write fuses and bootloader

avrdude -c usbtiny p atmega328p -U lfuse:w:0x62:m -U hfuse:w:0xD9:m -U efuse:w:0xFF:m -U flash:w:bootloader.hex

Upload new firmware

avrdude -v -p atmega328p -c arduino -P /dev/ttyUSB0 -b 115200 -D -U flash:w:blink.hex:i

Optiboot for 2.4576MHz crystal:

cd ~/app/arduino-1.7.10-linux64/hardware/arduino/avr/bootloaders/optiboot
vim Makefile

add

	atmega328_245: TARGET = atmega328
	atmega328_245: MCU_TARGET = atmega328p
	atmega328_245: CFLAGS += '-DLED_START_FLASHES=3' '-DBAUD_RATE=115200'
	atmega328_245: AVR_FREQ = 2457600L
	atmega328_245: LDSECTIONS  = -Wl,--section-start=.text=0x7e00 -Wl,--section-start=.version=0x7ffe
	atmega328_245: $(PROGRAM)_atmega328_245.hex
	atmega328_245: $(PROGRAM)_atmega328_245.lst

	atmega328_isp_245: atmega328
	atmega328_isp_245: TARGET = atmega328
	atmega328_isp_245: MCU_TARGET = atmega328p
	atmega328_isp_245: HFUSE ?= DE
	atmega328_isp_245: LFUSE ?= 62
	atmega328_isp_245: EFUSE ?= FF
	atmega328_isp_245: isp

afer make, flash optiboot_atmega328_245.hex using

avrdude -c usbtiny p atmega328p -U lfuse:w:0x62:m -U hfuse:w:0xD9:m -U efuse:w:0xFF:m  -U flash:w:optiboot_atmega328_245.hex

At the end of ~/app/arduino-1.7.10-linux64/hardware/arduino/avr/boards.txt add

##############################################################
my245MHz.name=ATmega328 2.4576MHz
my245MHz.upload.protocol=arduino
my245MHz.upload.maximum_size=32256
my245MHz.upload.speed=115200
my245MHz.bootloader.tool=avrdude
my245MHz.bootloader.low_fuses=0x62
my245MHz.bootloader.high_fuses=0xD9
my245MHz.bootloader.extended_fuses=0xFF
my245MHz.bootloader.path=optiboot
my245MHz.bootloader.file=optiboot_atmega328_245.hex
my245MHz.bootloader.unlock_bits=0x3F
my245MHz.bootloader.lock_bits=0x0F
my245MHz.build.mcu=atmega328p
my245MHz.build.f_cpu=2457600L
my245MHz.build.core=arduino
my245MHz.build.variant=standard
my245MHz.build.board=ATMEGA328_2.4576MHz
##############################################################

What are lock bits? :

my245MHz.bootloader.unlock_bits=0x3F
my245MHz.bootloader.lock_bits=0x0F

I copy these two lines from another Arduino board. What are right lock fuses?

Is all ok? Fuses, optiboot compilation, and arduino board settings?

Thank you!