Using furthest down Cell for math statements

[ssword]

I have printer toner usage sheet that has dates in a column I want it to reference the furthest one down to complete its job. I've attached an example excel file of what I've to to make it work but its dirty and doesn't work properly if the value changes to 100%, if I change the cartridge. For the most part what I have works but its terrible and difficult to alter.

What I'm trying to get it to do is find the average Page Per Day printer from the top value OR if column C has a cell with 100%. Additionally, I need it to references the furthest cell down that isn't 0 or nothing to find out which date to use to figure replace by date and the toner empty date. I'm hoping for a more elegant solution that i'm currently using that is accurate even if the toner level goes up.