Sump Pump float above or below the French Drain level
Doesn't sound like a French drain (which typically describes gravity-driven drains that daylight away from the foundation) so much as a conventional drain tile loop connected to your sump  pail. 

Most sump pump floats only let the water get halfway up the pail's height or so. I wouldn't want it getting much fuller than that, personally. If you let the water level get above the drain inlet, the entire drain loop is now full of water. You have essentially no margin for error before your basement floods. 

I've had cases where my float stuck and the water level got very close to the top of the slab. Because so much water is now present throughout the drain system, the pump has to run for 10 minutes or more to catch up. That's an uncomfortable situation to be in.