How to specify a Basepoint for a block?

I have two equally sized parallel lines with the same X coordinates that I want to make into a Block.

I need its basepoint to be, horizontally, at the same point as any line's beginning; and vertically at the midpoint between the lines.

How do I find it, without using helper objects?

Thanks.

You use the 'mid between two points' option in object snaps. So while in the block command, picking the base point, hold Shift down, hit right mouse button, pick the 'mid between two points', then pick the endpoints of the two lines.

For some users the shift right click mechanic is confusing until practiced a few times. You can also start your line command, type in 'm2p' and hit enter. Then you choose the two points from which you are going to derive your midpoint from.

Thanks.