Assume you buy 15mm of a bond at par with a 4.5% coup at a price of 100

Assume we put up 10mm and borrowed 5mm to pay for bond

I would like to be able to adjust my cost of borrow...so effectively I am earning 4,50% on 15 mmand paying some cost on 5mm...I would like to be able to change the cost of borrow and then solve for my return which would be the coupon I earn minus the cost of borrow

Finally I would like to calculate a "breakeven"....Effectively looking at how much I earn (coupon X15mm - cost of borrow x5mm) for 5yrs...then determine how much the bonds can decline in price for me to have a zero return.So that would be initial trade 15mm minus the 5mm I borrowed minus my net earnings should come up with a value...which would be the price I break even at? what is the formula i will need to use when doing this problem.