I own a 421. I use a earthway 2750 for small grains and ocassionally for fertilizer and lime. The 421 is perfect for clover. It can spread like 5lbs an acre or so. About impossible to get that low with any other spreader I have used.
Like brian said 2 pass method works best for hand spread seeders. You can practice in your yard with turning rate and walking. So many cranks for 50 yards of travel. Then, take a garbage bag and spread the seed into it in the garage. Get that speed you get while walking. Do 100 turns and weigh.
Far as spread goes, it does not broadcast as far as other spreaders. I get 6-8ft radius with clover. Probably 8-10ft radius with buckwheat and grains. PRacticing spread wdth on a driveway or road will give you a ballpark.
421's hopper is a bit smaller than the earthway. I have it set well for clover spreader, so I leave it there. No-till throw n mow with grains, I just mix the clover with the grains. Tillage on a spot, I mix grains with clovers, then stir it in with a tire drag, spring harrows, or cultipacker. Then i topdress with a single pass of the 421 to make sure some of the clover isn't too deep.