I wish I could help Tommboy. But that's why I grow in organic living soil, too much doctoring around with bottles and back and forth measuring things. The growers who have got it sorted have generally have reached a point of intimately understanding their plants in the environment they have for them, and are aware of their needs and the nutrient lines they are using thru that experience, but for me it would be like going to a supplement shop and trying to buy a bunch of capsules for my nutrient/eating needs instead of opening the fridge and choosing what I need from real food.
The lower leaf yellowing is probably fairly normal at this late stage, known as senescence, it is tricky knowing when to add more Nitrogen, easier when the plant is a long way from finishing to add some. But towards the end you have to decide if it is really needed or not, I have been considering the same for my own plants in similar later stage flowering but I am currently choosing to hold off thinking that they can utilise the lower leaves and draw some nutrition from then if they to.
My understanding of leaf tip burn is it is usually from an excess of added nutrients, I don't know how that tallys/coincides with the feeding you have been doing. I am sure Shed will be able to give you a reply based on far more experience than me!