For Q1 and just for clarity, change the word "amount" to the word "percentage" and we're in agreement. Then it will read, "2106 Home Office- (3/12)*$7.5k=1.875k in interest * Percentage of house used". But you're going around your elbow to get to your thumb that way. TaxGuyBill's way achieves the same end result and it's a heck of a lot simpler.
For Q2 - What TaxGuyBill said.
For Q3 - What TaxGuyBill said.
