"it's not accurate"
You're kidding, right?
How "accurate" do you need?
And how "accurate" are you willing to risk your life on?
I would never interpolate, I'd always use the higher value. Then add 10% for whatever, then add 10% for the condition of the runway. (I've never seen a runway that was in "perfect" condition)
I have yet to run off the end of a runway, but I have seen people run off the end of the runway, then tell everyone that the "chart said we'd make it".