30cm is 1ft - the length of a school ruler =)
Invest in a tape measure - I was thrilled when I put the height increments on the jump wings my OH made me - I thought I was jumping around 70cm, turns out it was 95!
I'm rubbish at guessing height though, I was trying to decide which SJ classes to enter next weekend and kept saying to OH "show me how high 75cm is again?" *OH dutifully looks up from his book and gestures* "this high". I say "ok, thanks.... So, how high is 90cm?". He got a bit cheesed off with me after ten minutes of stupid questions about length conversions from inches-cm, and he fetched his tape measure, chucked it to me and told me to shut up! Lol