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You have the general idea right, but you are missing a few things. First, your calculation is ignoring ambient temperature. You must subtract that from your max temperature. Also, you should probably include a thermal interface material, which you will want to use between the LED board and the heat sink.
There are a few important pieces of the puzzle that got missed; most notably accounting for ambient temperature; unless you’re working in a 0°C freezer you need to subtract the temperature of the working environment off the 90° figure.
One should also account for the interface and package thermal resistances of the LED package itself, which might easily add up to 1°C/W or thereabouts.
The °C/W calculation tells you what the temperature rise will be above the surrounding air, so, the higher your air temperature, the less power you can handle.
For instance, if you think it might get up to 40°C (104°F), then you would subtract that value from 90°C first. This leaves you with 55°C max temperature rise under those ambient air conditions.
Adding a thermal interface material between the LED board and the heatsink will add another thermal impedance, which may seem counter intuitive, but if you don’t, there will be a microscopic air gap between the heatsink and the LED board which will have a much greater thermal impedance.
I see Rick has given a link to a valuable resource. You should definitely check that out. It gives a thorough and graphical guide to figuring out thermal impedance and temperature rise.
One final note is that running the LEDs at 90°C is still pretty hot. As a rule of thumb, LED life will roughly be cut in half for every 10°C higher the operating temperature. Lifetime is generally specified as having lost 30% of its output relative to a new LED, for a given drive current. So, the hotter you drive them, the more their output level will drop over time.
