The formula
tola_price_pkr = (lbma_usd_per_oz × usd_pkr × 11.6638 / 31.1035) + local_premium
One tola is 11.6638 grams and one troy ounce is 31.1035 grams, so dividing by 31.1035 and multiplying by 11.6638 converts the international per-ounce dollar price into a per-tola price. The local premium covers import cost, dealer margin and local demand in the Sarafa market, which is why the Pakistani board rate is never exactly the converted spot price.
Lower karats are derived from the 24K benchmark by purity: 22K = ×0.9167, 21K = ×0.875, 18K = ×0.75.
Sources: LBMA prices and data, All Pakistan Sarafa Gems and Jewellers Association, State Bank of Pakistan exchange rates.