MOSCOW, July 23 (Reuters) - The Russian rouble firmed against the dollar on Tuesday, helped by the approach of the corporate tax payments season when large exporting companies convert their foreign currency earnings.

By 1030 GMT, the rouble was 0.3% higher at 87.55 against the dollar. The corporate tax payments are due on July 29.

Another important support factor for the rouble is a scheduled meeting of the central bank on Friday, when it is expected to hike the main interest rate by 200 basis points to 18% to tame inflation and cool the overheated economy.

Trading in main currencies shifted to the over-the-counter (OTC) market becoming less transparent after sanctions on the Moscow Exchange and its clearing agent, the National Clearing Centre (NCC), were introduced on July 12.

The central bank, which is using the OTC prices as guidance for the official exchange rate of the rouble, announced on Monday changes to its exchange rate formation mechanism.

The regulator said it would exclude extreme pricing values from its calculations and take trade volumes into account. It also said it would use the previous day's pricing as guidance if, on a given day, it received data from fewer than three banks.

Against the Chinese yuan, which had already become the most traded foreign currency in Moscow before the latest sanctions were imposed, the rouble was flat at 11.97, according to an analysis of the OTC market.

The Russian currency strengthened by 0.2% at 95.44 against the euro.

Brent crude oil, a global benchmark for Russia's main export, steadied at $82.53 on Tuesday after falling for the past two sessions, as investors remained cautious amid expectations of plentiful supplies and weak demand. (Reporting by Gleb Bryanski Editing by Gareth Jones)