Tried both methods and came back to your idea..hihi
The lookup table (four frequency points, plus interpolation) does the job better.
For info about the si5351 clock updates, the chip waits for the 8th register to be written to effect the frequency change.
I wrote a method to do as you said changing both clocks first 7 registers then quickly writing the 8th ones. It reduces the clicks but not 100%. Plus the FWD power reading gets messed up when the ATU tunes.
So now I have low, medium and high power dialled up in the menu. Neat.
Thanks for your help.
73, John (VK2ETA)