Sump(i think its called) question.

how do you get a siphon to go at the same rate as you pump it back up without calculating the volume of water the hose you use to siphon can move at any time - or must you use an overflow system?

First you need to find out how many GPH your overflows are running, then get a pump around that number. I usually go bigger on my pumps than the gph that my overflows are putting out, that way i can adjust them up or down to get the right amount of flow.

What kind of overflows do you have ?? Also what size ??

For me i have 2x 1" PVC overflows and my pump is a Quiet One 4000 putting out 1017gph i believe.. And that works good for me.. I could go bigger on my pump if i really wanted to though. Also forgot to add, for me i like to filter my tanks around 10 times per hour.. But that's me..