Buying a new place requirement for "rollover of gain" went out in the late 1990's.
If single, you can eliminate $250,000 of gain; if married you can eliminate $500,000 of gain. Any gain above the elimination is subject to tax at Capital Gains Rates.
Formula
Contract Sales Price less expenses of sale = Net Sales Price.
Original Contract Purchase Price plus certain purchase closing costs plus improvement = Basis
Net Sales Price less Basis = Gain on Sale.
Mike (CPA)