Time to transplant?

Doing this is there a need to flush it before harvest?

Probably with autos and short flower cycles but not always with 10+ wk flower cycles.

It really depends on a lot of factors such as

• how well the plant was able to get the food from the soil (which has lots of variables in itself)
• amount of light they get
• vpd (temperature & rh)
• others ...

Ultimately if you don't see the nitrogen start to fade in the leaves two weeks before harvest you'll want to start watering with extra runoff. Much more than the rest of the time but not like you're trying to correct for over-nute. You're not flushing out salts and replacing the water, just gently leeching away the remaining food. I'd say maybe 10% runoff. You'll see the plant start drawing the food from its leaves soon after.

Conversely if you find your plants are hungry too early or you overwater too much you can top dress a little with the ffof and water it in.