TO-220 case-to-sink thermal resistance
Total thermal resistance from junction to air is  

Rja = Rjc + Rcs + Rsa.  

Where:  a = air, j = junction, c = case, s = (heat)sink
Rjc &amp; Rsa are reasonably fixed by component choices.
Rcs is somewhat more variable - see below. 

If your data sheet does not give junction to case thermal resistance (usually Rjc or similar)* then change manufacturers - this is one of the most fundamental thermal parameters and ALL manufacturers 'worth their salt' will supply it. You can look it up for a handful of TO220 packages to get a feel. 

Your comment about varying with heatsink suggests you don't really understand the subject. Finding one of the many many good tutorials on the net would be advised.

Thermal resistances are like electrical resistance - they can be added in series.Heat flows from junction to case outer, from case to sink (cia washer and thermal paste, and from heatsink to air. So you get

      Rthermal = Rjc + Rcs + Rsa

      Trise jc = Rjc x Watts etc

      Trise = Tjc + Tcs + Tsa


see prior paragraph for meanings.


Rjc is specified by the device manufacturer.
Rcs is set by thermal washer material, heatsink compound, pressure of mounting etc. It should be a minor contributor.
Rsa is set by heatink design and size, air flow etc.


Temperature rise = Rja x Watts.

Search eg www.digikey.com  for TO220 and look at a few Tjc ratings.

This datasheeet

http://ww1.microchip.com/downloads/en/DeviceDoc/22057a.pdf

gives an unusually  large number of Rjc and Rja ratings for various package versions of the same part and is a good starting point.
For the TO220 package Rjc is 2 C/W.

Rja here is from about 30 to 60 C/W but the heatsinks are "wimpy".
Large heatsinks with fans can be around 1 C/W - much less with much care.

A well heatsunk TO220 can manage Tja = 10C/W without too much effort.
But it can be far worse without due effort.

______________________

Edited:

The initial paragraph originally read as follows. It is evident that I wrote "case to sink" but intended "junction to case". 

If your data sheet dies not give case to sink thermal resistance (usually Rjc or similar) then change manufacturers - this is one of the most fundamental thermal parameters and ALL manufacturers 'worth their salt' will supply it. You can look it up for a handful of TO220 packages to get a feel.