Wiping encrypted tablet

[Guest doveman]

I need to wipe an encrypted tablet (Sero 7 Pro) before sending it back for repair (battery not charging) but I can't do it from CWM, because that can't access the encrypted data partition and if I select Factory Reset from Android, it just reboots to CWM which still can't access /data.
 
I guess normally it would reboot to stock recovery and pass the PIN/Password to that to unlock it and wipe it. I've re-flashed the stock firmware from CWM (XE-Sero7ProUK-factory-4.2.1-stock_v1.zip) but that doesn't replace CWM with the stock recovery (I'm not sure I want to do that until I've finished anyway, so that I've got a way back in if it goes wrong). I've never changed the firmware, only rooted it.
 
I found this answer but it doesn't explain how to find which partition is data http://android.stackexchange.com/questions/33398/cannot-factory-reset-after-encrypting but this is the output I get from various commands:

~ # ls /dev/block
ls /dev/block
loop0         loop4         mmcblk0       mmcblk0p10    mmcblk0p3     mmcblk0p7
loop1         loop5         mmcblk0boot0  mmcblk0p11    mmcblk0p4     mmcblk0p8
loop2         loop6         mmcblk0boot1  mmcblk0p12    mmcblk0p5     mmcblk0p9
loop3         loop7         mmcblk0p1     mmcblk0p2     mmcblk0p6     platform

~ # cat /proc/partitions
cat /proc/partitions
major minor  #blocks  name

 179        0   30535680 mmcblk0
 179        1       2048 mmcblk0p1
 179        2      34816 mmcblk0p2
 179        3      32768 mmcblk0p3
 179        4      65536 mmcblk0p4
 179        5       8192 mmcblk0p5
 179        6       8192 mmcblk0p6
 179        7     786432 mmcblk0p7
 179        8     524288 mmcblk0p8
 179        9       2048 mmcblk0p9
 179       10      32768 mmcblk0p10
 179       11     262144 mmcblk0p11
 179       12   28737536 mmcblk0p12
 179       32       4096 mmcblk0boot1
 179       16       4096 mmcblk0boot0

~ # ls -la /dev/block/mmcblk*
ls -la /dev/block/mmcblk*
brw-------    1 root     root      179,   0 Mar 18 14:51 /dev/block/mmcblk0
brw-------    1 root     root      179,  16 Mar 18 14:51 /dev/block/mmcblk0boot0
brw-------    1 root     root      179,  32 Mar 18 14:51 /dev/block/mmcblk0boot1
brw-------    1 root     root      179,   1 Mar 18 14:51 /dev/block/mmcblk0p1
brw-------    1 root     root      179,  10 Mar 18 14:51 /dev/block/mmcblk0p10
brw-------    1 root     root      179,  11 Mar 18 14:51 /dev/block/mmcblk0p11
brw-------    1 root     root      179,  12 Mar 18 14:51 /dev/block/mmcblk0p12
brw-------    1 root     root      179,   2 Mar 18 14:51 /dev/block/mmcblk0p2
brw-------    1 root     root      179,   3 Mar 18 14:51 /dev/block/mmcblk0p3
brw-------    1 root     root      179,   4 Mar 18 14:51 /dev/block/mmcblk0p4
brw-------    1 root     root      179,   5 Mar 18 14:51 /dev/block/mmcblk0p5
brw-------    1 root     root      179,   6 Mar 18 14:51 /dev/block/mmcblk0p6
brw-------    1 root     root      179,   7 Mar 18 14:51 /dev/block/mmcblk0p7
brw-------    1 root     root      179,   8 Mar 18 14:51 /dev/block/mmcblk0p8
brw-------    1 root     root      179,   9 Mar 18 14:51 /dev/block/mmcblk0p9

~ # df
df
Filesystem           1K-blocks      Used Available Use% Mounted on
tmpfs                   497264        48    497216   0% /dev
/dev/block/platform/sdhci-tegra.3/by-name/CAC
                        516040      8596    507444   2% /cache

~ # tail ./etc/recovery.fstab
tail ./etc/recovery.fstab
/system         ext4            /dev/block/platform/sdhci-tegra.3/by-name/APP
/cache          ext4            /dev/block/platform/sdhci-tegra.3/by-name/CAC
/misc           emmc            /dev/block/platform/sdhci-tegra.3/by-name/MSC
/staging        emmc            /dev/block/platform/sdhci-tegra.3/by-name/USP
/modem          ext4            /dev/block/platform/sdhci-tegra.3/by-name/MDM
/metadata       emmc            /dev/block/platform/sdhci-tegra.3/by-name/MDA
/data           ext4            /dev/block/platform/sdhci-tegra.3/by-name/UDA  length=-32768
/thirdparty     ext4            /dev/block/platform/sdhci-tegra.3/by-name/THD
/sdcard         datamedia       /dev/null
/external_sd    vfat            /dev/block/platform/sdhci-tegra.0/by-num/p1

~ # tail ./etc/fstab
tail ./etc/fstab
/dev/block/platform/sdhci-tegra.3/by-name/CAC /cache ext4 rw
/dev/block/platform/sdhci-tegra.3/by-name/UDA /data ext4 rw
/dev/block/platform/sdhci-tegra.3/by-name/APP /system ext4 rw
/dev/null /sdcard datamedia rw
/dev/block/platform/sdhci-tegra.0/by-num/p1 /external_sd vfat rw

 

Clearly /dev/block/mmcblk0p12 is the biggest partition so is it safe to proceed using mke2fs -t ext4 /dev/block/mmcblk0p12 ?