I will be posting on the front page of our site silk updates and for other silk questions please email me - thank you.
ScurvyFreedman Has answered your question well.
A few thoughts - it is important to actually measure, as some gentlemen have long or short upper torsos which can really change the measurements.
Also the big end of your tie should be where you like it not where it makes a nice knot.
Generally a half-Windsor needs an extra 2 inches...