It was 224, I knew I was close, but not quite right!
Can I ask why you bootload them with the Nano, rather than the Uno or Pro Mini.
I've not had dealings with any other bootloader other than my own creation to be honest. It cost me around £5 to buy the parts (excluding the Uno that it plugs into).
Strictly speaking, I don't bootload at all. Using ICSP effectively overwrites any bootloader that is present (unless you're loading a bootloader 
) I did all my prototyping using a Nano, so my custom board is based on the Nano schematic, and I generally treat it as such.
I believe the Pro Mini is pretty much the same as the Nano, but without the USB interface - certainly the same processor. I think you can use the same bootloader on Uno, Nano & Pro Mini. I've certainly loaded the Uno optiboot bootloader on a Nano without any problems. I've always found my sketches run equally well (or badly) on the Uno & Nano. I assume the Pro Mini is the same (not sure about the extra couple of Analog pins).
As it happens, I've moved my project over to the Teensy 3.1 platform now, so will be running off a new set of boards in the not too distant future.
Cheers,
Ian.
DS3231 library:
Should this capacitor connected to AREF be connected to the GND? I always saw that it should be connected to the 5v. The question is just to know if I need to change my schematic
One side of a 100nF cap to aref, other side to ground.
You often see aref connected to vcc (and another cap between vcc and gnd), but this is wrong.