The best indicator to determine the minimum distance between WiFi and BT antennas is the antenna isolation and envelope correlation coefficient. My best recommendation would be to create a prototype board using your current board outline and experiment with placing the antenna on the board orthogonal to each other and then measure S21 on a network analyzer until you get at least -10dB of isolation between the two antennas. Then measure each antenna in an anechoic chamber to determine the ECC. anything less than 0.3 is great. I would not stop configuring the placement of the two antennas if the ECC is higher than 0.5.