plating allowance for M10x1.5 external thread with zinc plating

You can acquire this information yourself. First, you have not stated what plating you are talking about. Some plating processes have almost no plating build up or only .0001-.0003" per surface some like hard anodize can have as much as .0007". You can specify to your plater what tolerance you want him to hold within the parameters of the type of plating you are having done. Once you have determined what range of plating build up you are going to have, you can adjust the pitch diameter of your thread to compensate. Look up the thread in the Machinery's Handbook and find the tolerance for the thread you are using. The amount of tolerance you have is reduced by the amount of tolerance you give your plater.