On Locos site he says that you can add 4 turns of preload on a coil shock, does each turn equate to around 10lb?
Depends on the strength of your spring and the preload thread pitch.
400lb and 2.8" length of free stroke
You need to do the maths. And preload is measured in mm not turns. Measure the free length of your spring. If you wind preload on so it's 2.5mm shorter, you've added roughly one tenth of an inch of preload so you've added one tenth of your spring rate. In your case, it would make it 440 lbs at the start. To compress that 2 inches would need 400 x 2 plus the 40 of preload. Making 840 lbs. If say, you got a 425 spring and ran no preload, it would then take 850 lbs to compress 2 inches. So this option would be softer in the start of the travel and firmer at the end. You just pick what suits you better.
All this is assuming your linkage keeps the movement linear. It's a bit of an over simplification but you get the idea.