Before you read, know that I searched and searched and didn't find the answer to this question.

I know if you are running the stock blow off valve, you should tap the vaccum line off the BOV for a mechanical boost gauge. But, what if you are running a aftermarket BOV, especially 100% atmospheric? Will this location still give accurate readings or is there another vaccum line that you could tap instead?

the vaccum line is still the same whether it is a oem diversion valve or an aftermarket atmospherically vented BOV. Use the same line for the boost gauge, aftermarket BOV or not. If you are looking for accuracy then get an electronic gauge not a mechanical one!