I don't think a Workmate is heavy enough to sustain decapping and especially priming anywhere near the edge of the top. It will tend to tip and move around the floor. Build yourself a ~2' x 3' (or whatever x-y dimension works for your needs and space) bench with a 3.5" thick top (2 x 4's sideways clamped and glued) with a shelf below about 4-6" off the ground. Stack your loaded ammo and bullets on the shelf and it will sit there like big rock while you rock 'n roll on the top with your loading levers.