When I was on turboprops, we used to convert IAS>TAS roughly by multiply 3 times the tens of thousands of feet altitude. So at 10,000', the addition is 3 x 10, at 15,000', 3 x15=45kts increase for TAS. It used to work pretty well (I think, without decent navigation systems, you never really knew where you were, so the speed was pretty immaterial). It used to make dodging the Russians up the Berlin Corridor 'interesting'.