I'll second 'gaw'. I've got an older Keystoker 90 DV that I've just become the new owner and fully rebuilt. In conversations with Don at Keystoker, he mentioned -0.04 draft as the target, but I have a difficult time getting it above -0.02 or -0.03.
At the same time, my initial trials proved very successful, and I was sure to seal the grate well at the rear gap and also the sides up 'til the first row of holes.
I had my pusher mis-adjusted and it wasn't feeding coal properly, so the fire started to die out, stopping at the last holes on the grate, with NO indication of burning back to the hopper (which currently has no lid).
The older units, like mine, had the combustion blower wired to the stoker motor. So, unless it was stoking, it had no air from the blower and relied on natural draft to keep idling. That would require the -0.04 to -0.06 spec. that is in the manual. But, I've rewired mine to run the combustion blower all the time like the newer units, and really you only need enough draft to keep negative pressure in the stove at all times.
I'd check all the grate joints and re-seal them with some good furnace cement. Even better, pull out the grate and clean under it well. Make sure there's no ash/coal buildup under it. Then re-seal it.