A tick is a price increment. See
http://help.betfair.com/contents/itemId ... ex.en.html for the list of acceptable odds to Betfair.
Say you were manually entering a bet, you would need to click the down arrow 51 times to get down to 1.5 therefore it is 51 ticks lower.
When you "say 2.02 down to 1.5" do you mean if you lay it first and it's gone the wrong way?