Generally build out to an hour TiZ, but realise it needs to be done in smaller steps than sweetspot.
Might be worth doing a Kolie Moore style TTE test to start and use that number as a start point for TiZ and build from there. I think when working very close to FTP it takes more experimentation to find your individual start/end points and se how much progression you can take each week. I also prefer to avoid ERG for these and do them with a resistance setting and more by feel.