How about creepage distance?
OK That depends on what actual voltages you expect to up to. Keep your pads 1-4, HV on 2+3 as skinny as possible,
don't route tracks between 1-4 and 5-8 side any closer to 2+3. Have a spare TEST IC PAD or two on the PCB permanent,
and run a 250V / 500V megger test before assembly. I've used Kapton tape under the IC, ie a long strip right down the
length of the PCB as I run 22X in a row (in my BM modules). If you DO expect to hit + and - 250V on adjacent ICs,
then, do the lot, HV coating and micro grooves / routes. The max I design to is 200VDC between any pins, and rarely
ever need to go to any extra lengths (besides skinny pads etc). A megger is your best friend !!!
Edit: I've been making BM pcbs for several years using this IC and have never had a flash-over yet ! (Touch wood?)
Edit2: Given the alternate definition of wood, is the expression "Touch wood" appropriate any more
??