Not really looked into the other figures (-1,-2 etc...) but the equations are all pretty linear - take an educated guess and you won't be far wrong.
As far as the total wind speed goes, it assumes an 'effective' windspeed. This means that if you are doing 40 into a headwind of 20 then the average effectve speed would be up to about 60 or so. Funnily enough, this does not translate in the opposite direction, mainly because of the way low pressure builds up at your back at speed. Therefore if it were a 20 MPH tailwind, you would not experience an effective 20 MPH wind - it would most likely be more than that.
Also, taking into account sidewinds would be a nightmare because of the inherent number of variables introduced - you can do the math, not me!
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