Headspacing isn't the issue, bullets touching the lands is. OAL is what we're discussing. With barrels that have short throats and cone shape bullets like Berry's 124 HP you have to load to a shorter OAL.
I would break down the gun so I had the barrel by itself. After seating a dummy round you should seat a bullet long and work your way down in length. Drop the bullet in the chamber, If the bullet is sticking where you have to pull the bullet out, then you are too long. Keep seating it deeper into the case until the bullet drops in the chamber and it should fall right back out
That's how I do it too, except I try to twist the round in the chamber. That gives a better feel for the lands when the bullet is just barely touching them. When you're off the lands the cartridge will spin freely in the chamber with little effort.