If i'm not mistaken I believe the sump and the tank are hooked together, water flows into sump from MT and then from sump to MT. If the MT is cycling so will the sumps water because it is the same water. But the LR cultivates bacteria that breaks down the bad stuff (ammonia, nitrites, and doesn't allow nitrates to accumulate like bio media such as bio balls and bio rings), so I would think the more LR the better! and you are correct, 80lbs of LR will probably be to much for the MT, speaking as if I were you which I'm not I would add 60 lbs of LR to MT or however much is needed to make your tank look the way you want it to and then add the excess to the sump which will allow maximum LR filtration without taking up the volume of your MT. Depending on how many sections you have in your sump you could go with a few diff setups for maximum removal of the bad stuff to maintain the water quality. Personally I wouldn't add any bio media if your going with 80 lbs of LR. In my tank I have 0 bio media. As to the sections you could have the first section containing the charcoal, skimmer, heater, pumps and all the equipment and depending on how the water flow is working in the sump you could set it up so that the water flows into the second chamber at one central point via hole drilled in the divider with a pvc pipe in the driller hole that's wide enough for ample water flow, then you could stick a mechanical filter in the pvc pipe for collecting unwated materials while allowing water flow. And if you have room for two more sections you could add chaeto into the second chamber, which would be a small chamber since chaeto doesn't really take up all that much room (chaeto would help big time of the removal of certain bacterias), then the third chamber could hold some substrate and your LR and allow you to grow corals and foods on the reverse light schedule that you'll have setup for the sump, you kinda need LR for cultivating things so that critters have places to hide and corals have places to be attached too. I would think that would be the way to go if your going to do a DIY sump. Again that is what I would do if I were you but it is your tank so do you what you want! As for the main tank I would do a sand bed in between a normal sand bed and a deep sand bed to conserve volume and then add the extra amount that you want to that last chamber so you will still get the benefits of having that amount of substrate and LR for filtration and still allowing your MT to have the volume that you want.
But again I don't believe it matters in what order you have your filtration setup because the water is circulating through the tank and the sump and will get the same benefits at the same rate no matter the order of your filtrating set up. Say you had 35x turnover per hour in your tank. That means your pumps are circulating the water of your tank 35 times an hour. So basically the water will have run through the filtration process 35 times and the order of your filtration setup would have no affect on the water quality. More turnover rate means the more times your water has had the chance of going through the filtration setup, so the more the better as long as your MT isn't receiving to much water flow to the point that a hurricane is sweeping through your tank, corals and animals only like so much flow.
Hopefully that might help out some in your planning! I would write all your plans down along with small sketches so that your really able to plan it out correctly and correct errors in your DIY items. Just make sure you understand the process that the water will be going through (not saying you don't and that I do know, some of the above might be incorrect but I'm pretty sure that's how it works someone please correct me if i'm incorrect so that Steve can have the best possible tank) Good luck to you man.
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