I am using an exhaust with forced draft. I have a conical funnel (8" back 12" front) above the fire. A 6"pipe (Yes sir only 6") up to 6" below the ceiling and then horizontally (I did not want to take hole in the roof) out of the building. At the bend -Tee actually - I blow air from an old vacuumcleaner. This way I get a kind of ejector that pushes the smoke out. A real exhaust fan for hot smoke is far too expensive for me.

All air that leaves the shop goes through this funnel. All smoke is sucked into the funnel and pushed out through the wall. I have zero smoke problem.

This works perfectly if I have the doors shut. As soon as I open a door, the resulting draft pushes the smoke to the side so some of it escapes the funnel and goes into the shop. If I were to have a second exhaust in the shop, that exhaust would pull the smoke back into the room.

As I see it, the important "ventilation" is to get the smoke out. This means to have the exhaust close to the forge and no other air exit that competes. To put in a fan that pushes extra air into the building will of course improve the situation as long as it does not disturb the smoke from going into the funnel - an exhaust fan in the wall will pull smoke back into the shop.

I do not believe that my ejector makes all the difference a good vertical flue would work the same way.

I realize that your situation is not my situation but this is what works for me.

Cheers

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my first picture

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It worked!   This shows my funnel with new coals added to show smoke

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