I cleaned the tank screen on my 300TD a while back. I ran the tank down as far as I could, I still had about 2.5 gallons that came out of the tank. Mostly into containers, with a fair amount on me. Since the tank lays down in the wagons you can still have quite a bit of fuel in the tank when it seem there isn't much.
I was changing my fuel lines, so I let a lot of it drain through the hoses, but I also loosened the tank screen, it's on the bottom, and drain a lot that way. If you have a large bucket to catch fuel in I would just pull the tankscreen and let it drain. Just loosen it so a little comes out not all at once. This way you can close it again when your bucket is full. Make sure you have extra buckets on hand there may be more than you planned on.
If you drain is through the hoses, take the cap off the tank so it can breath. I didn't and got the glug glug draining which is slow.

You can just clean the screen off. Just make sure it's dry before you put it back in. Make sure you put on a new o-ring. Apparently they tend to leak if you don't, and you don't want to have to do it all again, and the o-ring is cheap.

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