Sorry about the rulers … we intended to also hand out rulers but as Jose mentioned any ruler will do and should be a null issue. If you absolutely have no access to a ruler PM me your address I will Amazon one to you.
Remember it’s about total growth so even if your ruler starts at 30 CM, as long as you always measure starting at that mark, it won’t make a difference.
As a rule of thumb, yes please use the CM side of the ruler and measure from base of plug or rock if you removed it from the plug.