If you are using the ASME Y14.5 standard, there is no default. You have to have a note stating whether the dimensions and tolerances apply before or after plating unless otherwise specified. So if you don't have any notes in your drawing, then your manufacturer/supplier can/will interpret your drawing the way they prefer.
Sometimes, instead of direct notes, companies have their own "drawing interpretation" specifications which are referenced on the drawing and which specify whether dimensions and tolerances apply before or after coating.
Smart people have a note on the drawing stating that dimensions and tolerances apply after finishing. Finishing probably will not affect the perpendicularity, but it can affect your accurate tolerances.
You can work out the effect of finishing, correct your tolerances accordingly, and order the part without finishing. You send the part out for finishing, with instructions on how much material can be removed or added. You will quickly see why few people do it this way.
Well, it depends on whether you use one or two vendors for this item.
If you use one vendor, you control geometries of your finished item (with plating) and specify plating thickness as a specs.
If you use two vendors, then you will have a print for item before-plating, and another print for after-plating. On the 2nd print, you can control either plating thickness or finished sizes. But you don't want both.
From experience, unless you have any reason to, it's always best to note tolerances apply after finishing and to use one supplier to sub-contract the plating. Even better, explicitly state on order that you require a certain plating thickness to be considered during manufacture. Most will often ignore this and just manufacture to drawing. You have some very tight size tolerances there that plating, not knowing your expected process, could affect greatly.
