I started with something like this, Advaneced Heat Vents and Exchangers to get the Overclocked Heat Vents at the edges cooled. But I wanted to get that little bit of excess heat cooled, so I played around with the simulator. It was a bit tricky and I don't claim to understand it entirely, there seems to be a difference between the top and the bottom of the reactor so the components behave differently there, but I got it running and I'am happy with it (for now ^^).
I changed out the Advanced Heat Vent in the bottom right with an Overclocked Heat Vent; and the entire design instantly balanced out into a Mark I EA with a 4.29 of 4.29 EU Efficiency. Heat Vents are funny things; They behave differently depending on where and how you use them.....
Later .... Peace ....