The order of operations appears to be quite different in your second calc compared to your first. Should it be like this:
( -23.6 +
( 5.01 * Ln ( Curing + 0.001 ) +
( 0.0281 * Temperature ) -
( 0.266 * Sqrt ( Relative Humidity ) +
( 0.633 * Sqrt ( Wind Speed ) )
Seems like an ideal case for using the Let() statement so that the complex formula stays understandable...
Shortly after I asked the question, I found the answer was in misplaced parts of the statement. It now works very well. Thanks for your help.