Typically, the drying + curing of WB inks goes through 2 distinct phases; 1^st removing the water + 2^nd curing the ink.

In the tropical north, the magnitude of the 1^st task assumes much greater proportions. Typically with drying, you’re letting the water evaporate from the print. Unfortunately, under humid conditions, the fabric is absorbing a HUGE amount more from the atmosphere, so you may have 100 x more moisture to remove in the heat press. The good news is that you have a heat press, so moisture per se is not as big an issue as if you had just a drying tunnel.

The only way to be sure that you’re actually curing the print is to do a test wash. Our guess would be that, if 40” is getting you close, then another 40” will probably end up cooking it. That is, too MUCH heat is every bit as bad as not enough. If you’ve done a test wash and it’s not enough, then just try an extra 10-15”, rather than the full 40. You could also keep a work book in which to record results; these were the conditions used + these were the results. It may be a pain in the short term, but will ultimately grow to be an incredibly useful reference source.