Hi Zora,
Her mane is 9cm 2mm from forelock to about 1cm from the withers where it extends to 10cm
I've added a couple of pics where I have a comb in her hair to show you the line of the base of the mane. one of them is just with the comb in, one I am holding the comb down to apply even pressure so the hair lies flat from the nape of the neck.
I have just realised these pics show her with her mane tucked behind her ear. If you need other detail pics let me know, but the light is going her so it will probably be tomorrow before I can take them.