How do I obtain the thermal resistance of the heat sink?

Fig. 1 shows the thermal model and the name of each part of the package. Each symbol represents as follows.

Pd: IC power dissipation
Ta: Operating Ambient Temperature
Tj: Junction temperature
Tc: case temperature
Ttab: tab temperature (temperature of the heatsink of the audio power IC)
Rth(j-c): Thermal Resistance between Junction-case
Rth(j-t): Thermal Resistance between Junction-tab
Rth(c-a): Thermal Resistance between case-Ambient
Rth(HS): Thermal resistance of the heat sink (external heat sink)

This section describes the procedure in detail to obtain the size of the external heatsink, whose Rth(HS) (thermal resistance of the external heat sink) makes Tj be less than 150 °C, when the output power per channel is 4W and Ta is 40 °C for HZIP25 products

First, read the power dissipation when the output power per channel is 4W from Fig. 2. (Pd=15.3W)

Next, find the required Rth(HS) value of the thermal resistance of external heat sink by using the following equation.
The thermal resistance between junction and tab, Rth(j-t), of HZIP25 package is 1.0°C/W as shown in the data sheet.

Tj > (Rth(j-t) + Rth(HS)) * Pd + Ta
150 > (1.0 + Rth(HS)) * 15.3 + 40
Rth(HS) < 4.88 ℃/W

Then, read the required size of the external heat sink by using Fig. 3.
It can be estimated that 121cm^2 and more are required for 2mm aluminum plate.