It's hard to overstate just how shit the tallow-based Midnight Stag soap is. I took thrice as much as I would with a normal soap, and still got a foamy, airy "lather". It is slick, though. If it weren't such a meme scent...
I think the soap is fine but certainly not top-tier. After using Midnight Stag for my first AA, I realized it is not really a great summer (northern hemisphere) scent for me.
With that base I remember the proto-lather never really got any "bigger" while lathering, so I tub loaded what looked like it was going to be enough soap for the whole shave, then just added water to weigh it down a bit. That's pretty much it.