Pay off mortgages or use interest as tax deduction?

[Russell Buxton]

I have 4 mortgages where I am paying approx. $35k per year in interest. This obviously reduces my tax burden. With other deductions, I pay no taxes at all. 

I have the funds to pay off these mortgages.  Would it be wise to do so? How can I calculate the 'sweet spot' between the max. amount of interest to owe-v- least taxes to owe?

e.g. Let's  say I pay down the mortgages to only having $20k in interest paid each year.  How would I calculate if my tax burden is $15k or less? The point being, if I have no interest payments, what % of the 35K is likely to just go directly to higher tax payments assuming all other variables are equal?

Thanks

Side note: I am aware of the variable that the opportunity cost of the return I could get on my funds if I used them elsewhere would be lost.