Electronic – Needed PCB surface to cool a SOIC-8 EP package

You don't use the original Tja anymore because it's not valid - there are new thermal resistances to place between the junction and the ambient air.

Have a look at this Analog Devices application note MT-093.

You can see that: $$\Theta_{JA} = \Theta_{JC} + \Theta_{CA}$$

Now when you add a heatsink, you are inserting another thermal resistance between the case and ambient air, so your equation becomes:

$$\Theta_{JA} = \Theta_{JC} + \Theta_{CS} + \Theta_{SA}$$

Where Tcs is thermal resistance of the case to your sink. This depends on your interface material, sometimes known as "thermal grease". Various factors (surface area, applied pressure) change how well the thermal grease will conduct heat. An estimate of .25°C/W seems reasonable.

Tsa is the thermal resistance of the sink to ambient. (3°C/W as stated, with some airflow)

So to calculate your maximum allowable ambient temp:

$$ T_A = T_J - P * (\Theta_{JC} + \Theta_{CS} + \Theta_{SA}) $$

According to the note just below the thermal data table from that datasheet:

For packages with exposed thermal pads, such as QFN, PowerPAD™, or PowerFLEX™, \$\theta_{JP}\$ is defined as the thermal resistance between the die junction and the bottom of the exposed pad.

The way I interpret that:

So if you are attaching the heatsink in the normal way, on the back, against the pad, use the junction to pad thermal resistance.