General: Lower (left side) Fuel Sender

[Gazzaa]

Evening all,

Quick question - Anyone know the minium fuel level in order to remove the lower (left side) fuel sender unit, without the fuel over flowing?

Longer story, if you want to read on - My 2005 Diesel XC90 went into limp mode the other day and refused to start again, recovered home! Turns out the lower level fuel sender unit is fubar but the gauge on the dash was still reading just under half a tank, so ultimately I ran out of fuel.
Priced up the sender, £129 at Volvo or £27 from Autodoc (Bosch unit) and they appear to be exactly the same part!!
Ordered and fitted (after a bit of faffing about) and hay presto, the guage is working again. 20litres of fuel added, bit of purging and boom the ode gal lives again! Checked for leaks, all seemed good....turns out 20litres isn't enough to get to the left side of the tank...another 46litres does though. I've now got a very very slight weap from the left side sender....it looks like i've not centralised the unit or the O ring I thought looked ok, isn't so, either way I need to loosen the lock ring, but I dont want the contents of the tank to gush out.
Normally I would expect to let the guage below half a tank but I'm not sure how the cross feed system works, does it use the right side of the tank first or the left or a combination of both? hence the quick question above.

Cheers for reading
All the best,
Gazza