I'd lift that height up a bit - more like 250mm or so to keep debris from blocking the balance pipe over time. As water flows through it will tend to build up sediment around the hole.
I wouldn't fancy feeding my washing machine with water off the roof, it will be contaminated with bird poo etc
Have you considered using a simple siphon to connect the tanks together ?
All you need is a pipe from the bottom of the first tank up over and down to the bottom of the other. Once you have filled the pipe with water to get all the air out, the water level will always be the same in each tank, without any need to drill.
I can assure you the water is crystal clear
I wouldn't fancy feeding my washing machine with water off the roof, it will be contaminated with bird poo etc
it will be soft too which means the washing machine will last longer and use less detergent.
I can't drink our water straight from the tap, it stinks of chlorine, I have to let it sit for a while until it's all gassed off. But equally I have drained a tank that has had seagulls round it and a dead one in the bottom, the water was black and stinking, it's put me off tanks of water that our feathered friends have access to, guano contamination isn't nice.The percentages will be so minimal that it wouldn't be an issue. You can fit a UV filter which will kill any nasties.
We have well water and mains and the mains seem fine until you do a direct comparison and it smells like a swimming pool.