Tinned chickpeas can be replaced with dried, first soak over night in cold water and then simmer in un-salted cold water for 40 – 60 minutes or until soft. You might have to skim of any froth as they cook and top up the water every now and then.
I think making humous is an ARTY taste form, trial and error but taste as you go, adjusting the lemon and tahini for more or less depending on taste, more olive oil for consistency and flavour, more water to lighten and loosen. Season to taste - generous with the salt to bring out the flavours.
Tip – Tahini is sesame seed paste, found in all big supermarkets and delis. I think virgin olive oi is too rich and over powering so basic light olive oil is just fine.